Neuroscience · Unit 5: Homeostasis and affect

Circadian Rhythms and Sleep

Your body keeps a day of about 24 hours even without a clock on the wall, and it switches between waking, NREM sleep and REM sleep several times a night. This lesson shows where the master clock sits and how lesion and transplant experiments found it, how a loop of clock genes keeps time and how light resets it, how a night of sleep is scored from EEG, eye and muscle recordings, and why loss of orexin neurons makes waking unstable. Afterwards you can compute free-running drift, score a hypnogram and design an experiment that separates necessity from sufficiency.

Watch first Light Resets a Slow Clock · 3:02

In 1972 Friedrich Stephan and Irving Zucker destroyed a small paired cluster of neurons just above the optic chiasm in rats, and the animals stopped drinking on a daily schedule.

The rats still drank, but no longer in a 24-hour pattern. That cluster is the suprachiasmatic nucleus (SCN), a nucleus being a compact group of neuron cell bodies in the brain. How does a group of cells keep time, how does light keep it on time, and how does the brain switch between waking and sleep?

The idea: a clock you can cut out and transplant

A circadian rhythm keeps a period of about a day with every time cue removed, so the timer is inside the animal. The SCN sits in the anterior hypothalamus (see the hypothalamus and autonomic control).

Two experiments made the SCN the master clock. Stephan and Zucker made bilateral electrolytic lesions in albino rats; SCN lesions eliminated circadian drinking and locomotor rhythms. That shows the region is needed, not that it generates time: a lesion also cuts passing fibres and could remove a relay.

Martin Ralph, Russell Foster, Fred Davis and Michael Menaker answered that in hamsters in 1990. They used the tau mutation, which shortens the hamster’s free-running period, as a label. They destroyed the host’s SCN, then grafted in SCN-region tissue from a donor of a different genotype. The restored rhythms ran with the donor genotype’s period, whatever the host’s genotype. A relay would have passed on the host’s period. A pacemaker imposes its own.

The SCN is not the only clock. Yoo and colleagues (2004) made knock-in mice in which the clock protein PER2 is fused to luciferase, so tissues glow in time with the clock. Isolated peripheral tissues such as liver sustained circadian oscillations for more than 20 cycles in culture. After SCN lesions, those tissue rhythms continued but drifted out of phase with one another. So the SCN keeps the body’s clocks in step. It does not supply their ticking.

The molecular loop and how light resets it

The genes came first from flies. Ronald Konopka and Seymour Benzer (1971) found mutations in one Drosophila gene, period, that made flies arrhythmic or gave them short or long periods (see CRISPR and functional genomics for how such screens work). Mammals use related genes in a loop.

In mouse cells, the transcription factors CLOCK and BMAL1 bind as a pair to E-boxes, short DNA sequences near their target genes, and activate the Per and Cry genes (Gekakis and colleagues, 1998; see transcription). PER and CRY proteins build up, bind each other and enter the nucleus. In reporter assays either CRY1 or CRY2 abolished CLOCK-BMAL1-driven transcription (Kume and colleagues, 1999). As PER and CRY are degraded, the brake comes off and a new cycle begins.

Why does that oscillate instead of settling at a steady level? Think of a shower with a long pipe: you turn toward hot, nothing changes, you turn further, and when the hot water arrives you have overshot. Negative feedback with enough delay keeps overshooting in both directions. In the clock, transcription, translation, modification, nuclear entry and timed degradation supply hours of delay. Signaling circuits treats delayed negative feedback as a circuit motif.

The loop runs near 24 hours, not exactly 24, so light must reset it each day. Light reaches the SCN through the retinohypothalamic tract, a tract being a bundle of axons within the central nervous system. Berson, Dunn and Takao (2002) recorded from SCN-projecting retinal ganglion cells in isolated rat retina and found that they still depolarized to light with rod and cone input blocked. Those cells express melanopsin (Hattar and colleagues, 2002), a photopigment that signals through a different G-protein cascade from the rod’s cGMP pathway (see the senses and cGMP and the retina). Rods and cones also contribute: mice failed to entrain to light-dark cycles only when melanopsin and rod and cone phototransduction were all removed (Hattar and colleagues, 2003). In the mouse SCN, light pulses strong enough to reset the clock rapidly induced Per1 transcripts.

Timing matters. In healthy people under controlled conditions, bright light before the core-body-temperature minimum (usually in the early morning hours) delayed the clock, and light after it advanced the clock.

Worked problem: free-running drift. Czeisler and colleagues (1999) measured a mean intrinsic period of 24.18 h in healthy young and older adults under controlled dim light. That is a study mean; individuals vary. Assume an idealized clock with exactly that period, no time cues, starting in phase with a 24.00-h day.

Step 1: excess per cycle. 24.18 − 24.00 = 0.18 h, which is 0.18 × 60 = 10.8 min per cycle.

Step 2: seven internal cycles. 7 × 0.18 h = 1.26 h, which is 75.6 min. The clock’s marker now falls 75.6 min later than on the 24-h day.

Step 3: check. Internal time is 7 × 24.18 = 169.26 h. External time is 7 × 24.00 = 168.00 h. The difference is 1.26 h, as before.

Step 4: what entrainment needs. To stay at 24.00 h, this clock needs an average net light-driven advance of 10.8 min per day.

Free-running drift — a 30-cycle actogram of one clock marker⚙ original · interactive
Excess per cycle (period − 24.00 h)
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Net drift per day (excess − shift)
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Marker offset at the cursor
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Check: internal time against external time
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To stay at 24.00 h this clock needs
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In this sketch: an idealized clock with a constant period, no time cues, starting in phase with a 24.00-h day; light is a fixed net shift per day, with no phase-response curve. The lesson names the timing of light but does not give it as a rule this sketch uses: in controlled human studies, bright light before the core-body-temperature minimum delayed the clock and light after it advanced it. 24.18 h is a study mean (Czeisler and colleagues, 1999, healthy young and older adults, controlled dim light); individuals vary.

Offset after n cycles = n × (period − 24.00 h); with a 24.18-h period that is 10.8 min per cycle, 75.6 min after 7 cycles (7 × 24.18 = 169.26 h against 7 × 24.00 = 168.00 h), and an average net advance of 10.8 min per day holds the marker at 24.00 h.

Reading a night

Sleep is scored by polysomnography: scalp EEG, the electro-oculogram (EOG) for eye movements and the chin electromyogram (EMG) for muscle tone. The night is cut into 30-second epochs, and each epoch gets one stage. In adults:

  • N1: the alpha rhythm of relaxed waking gives way to theta activity at 4 to 7 Hz.
  • N2: sleep spindles, trains of 11 to 16 Hz waves lasting at least 0.5 s, and K-complexes, well-defined biphasic waves lasting at least 0.5 s.
  • N3: slow waves of 0.5 to 2 Hz with peak-to-peak amplitude above 75 µV in frontal leads, filling at least 20% of the epoch. N3 merges what older scoring rules called stages 3 and 4.
  • REM: an EEG that can look much like waking, rapid eye movements on the EOG, and the lowest muscle tone of the night on the EMG.

So the EOG and EMG separate REM from waking. In rodents, glutamatergic neurons of the sublaterodorsal nucleus are thought to produce REM atonia by recruiting inhibitory neurons in the medulla and spinal cord. Dreams are not limited to REM: people woken from NREM sleep also report them (Siclari and colleagues, 2017).

Worked problem: scoring a hypnogram. In this made-up recording, a subject is in bed for 960 epochs. The scorer counts 60 epochs of wake, 48 of N1, 432 of N2, 192 of N3 and 228 of REM.

Step 1: total time. 960 × 30 s = 28,800 s = 8.0 h. The counts add to 60 + 48 + 432 + 192 + 228 = 960.

Step 2: sleep. 960 − 60 = 900 epochs, or 900 × 30 s = 27,000 s = 7.5 h.

Step 3: sleep efficiency, sleep divided by time in bed: 900 / 960 = 0.9375, or 93.75%.

Step 4: stage shares of sleep. REM 228 / 900 = 25.3%. N3 192 / 900 = 21.3%. N2 432 / 900 = 48.0%. N1 48 / 900 = 5.3%. They sum to 100%. These are practice numbers, not norms.

When you sleep is governed by two processes. A homeostatic pressure, often called Process S, accumulates during waking and dissipates during sleep. A circadian signal from the SCN, Process C, rises and falls with the clock. Sleep comes most easily when pressure is high and the circadian wake signal is low.

How does the brain switch? In the ventrolateral preoptic area (VLPO), sleep-active neurons release GABA and galanin and inhibit wake-promoting cell groups. Those groups (histamine neurons of the tuberomammillary nucleus, noradrenaline neurons of the locus coeruleus, serotonin neurons of the raphe and cholinergic neurons of the brainstem and basal forebrain; see transmitter systems) are inhibited by the VLPO and in the model inhibit it in return, though in slice recordings VLPO sleep-active cells were inhibited by noradrenaline, acetylcholine and mostly serotonin, not directly by histamine (Gallopin and colleagues, 2000). One model calls this mutual inhibition a flip-flop: whichever side is ahead silences the other, so transitions are quick and intermediate states are rare. It is a model. In mice, Chung and colleagues (2017) found that activating preoptic GABA neurons that project to the tuberomammillary nucleus increased NREM sleep, while activating preoptic GABA neurons in general increased waking. Projection target together with transmitter, not location alone, defines a sleep-promoting cell: glutamatergic preoptic neurons with the same target promoted waking.

Orexin and stable waking

A flip-flop switches quickly but can flip too easily. Neurons in the lateral hypothalamus that make the peptides orexin A and B (also called hypocretin 1 and 2) excite many of the wake-promoting groups, and are thought to hold the switch on the wake side.

Three lines of evidence tie orexin to narcolepsy, and each comes from a different species. In a familial dog model, Lin and colleagues (1999) found an autosomal recessive mutation in the hypocretin receptor 2 gene. In mice, conditional loss of orexin neurons in adults fragmented waking and increased transitions between sleep and wake states, and dark-period wake time was significantly reduced from 3 to 10 weeks after induction, with recovery by 11 weeks (Tabuchi and colleagues, 2014). In people, Thannickal and colleagues (2000) counted 85 to 95% fewer hypocretin neurons in four postmortem narcolepsy brains than in twelve controls, while neighbouring neurons making melanin-concentrating hormone were not reduced. Most human patients do not carry the dog’s receptor mutation; narcolepsy type 1 is associated with severe loss of brain orexin. An autoimmune attack on these neurons is hypothesized, not established. Cataplexy, the sudden muscle weakness many patients have, occurs with consciousness preserved, so it is not falling asleep.

Adamantidis and colleagues (2007) expressed channelrhodopsin-2 in hypocretin neurons of mice and delivered light through an optical fibre into the lateral hypothalamus of freely moving animals. Stimulation increased transitions from slow-wave and REM sleep to waking. Trains at 5 to 30 Hz shortened the latency to wake, while 1 Hz did not.

Design problem

A colleague says the optogenetic result and the human cell counts prove orexin neurons are needed for waking. Design a mouse experiment that asks whether they are needed for waking itself or for stable waking: what you measure, your controls, and your prediction.

One way to do it

Driving the neurons shows that their activity is enough to wake a mouse under that stimulation: sufficiency, not necessity. The human series is correlational: the cells are gone in four patients, but not when or why they were lost.

To test necessity, silence or ablate the neurons with an inhibitory opsin or a toxin expressed only in orexin neurons, and do it in adult mice so development cannot compensate. Include opsin-negative mice that receive the same light, to rule out heat, light and handling. Record EEG and EMG continuously and have blind scorers assign stages in fixed epochs.

Measure total wake per 24 h, the mean duration of wake bouts, the number of transitions between states, and episodes in which the EEG shows waking while the EMG shows atonia. If the neurons are needed for stable waking, you predict much shorter wake bouts and many more transitions with little loss of total wake, plus cataplexy-like episodes, scored as abrupt atonia lasting at least 10 s with a theta-dominated, wake-like EEG after at least 40 s of waking, with video showing immobility. If they were needed for waking itself, total wake would fall sharply.

Two limits remain. Orexin neurons release other transmitters, so silencing the cells does not isolate the peptide; a receptor antagonist or peptide knockout tests that. And the result holds for these mice and this timing, not for human patients.

What the sleeping brain does with its hours, including replay of hippocampal activity, is the subject of memory consolidation.

How we measure it

Lesion and transplant studies

Destroy a region and see what is lost; then replace it with donor tissue and see what returns. A lesion tests necessity but damages fibres and every cell type in its path, and a transplant with a genetically marked donor shows which tissue sets a property such as period.

Bioluminescence reporters (PER2::LUCIFERASE)

Fuse a clock protein to firefly luciferase in a knock-in mouse, culture a tissue with luciferin and count the photons it emits over days. The glow rises and falls with the clock protein, so each tissue's rhythm can be followed in real time.

Polysomnography (EEG, EOG and EMG)

Record scalp EEG, eye movements (EOG) and chin muscle tone (EMG) through the night, then assign one stage to each 30-second epoch. EEG alone cannot tell REM sleep from waking; the muscle and eye channels can.

Optogenetics

Express a light-gated channel such as channelrhodopsin-2 in one genetically defined cell type and drive it through an optical fibre; silencing needs a different, inhibitory opsin. Driving cells tests sufficiency; only silencing or removing them tests necessity, and either holds only for the cells, timing and behaviour tested.

Finish line

Test yourself

8 questions

Practice and MCAT-style, with explained answers.

Next · Lesson 22

Memory Systems

H.M. got better at mirror drawing over three days and did not remember ever having done it. What kind of memory improved?

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CLOCK-BMAL1 — the transcriptional activator of the clock loop
CLOCK-BMAL1 — the transcriptional activator of the clock loop ● LIVE · RCSB PDB 4F3L
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Loading live structure 4F3L from RCSB PDB…

Two partner proteins wrap around each other. Find the basic helices at one end, which in the DNA-bound model reach into the groove at an E-box (4F3L itself holds no DNA), and the PAS domains that hold the pair together.

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Orexin receptor 2 — the receptor mutated in narcoleptic dogs
Orexin receptor 2 — the receptor mutated in narcoleptic dogs ● LIVE · RCSB PDB 4S0V
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Loading live structure 4S0V from RCSB PDB…

Seven helices cross the membrane, as in every G-protein-coupled receptor. Find the antagonist folded into the pocket open to the outside, where orexin peptides would act.

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Ronald Konopka

Ronald Konopka

As a graduate student in Seymour Benzer's laboratory, isolated three Drosophila mutants with an arrhythmic, a short or a long circadian period, all mapping to one gene they named period: the first sign that single genes set the clock.

In the late 1960s many biologists doubted that a behaviour as complex as a daily rhythm could be changed by a single gene. Seymour Benzer's laboratory at Caltech was using chemical mutagenesis in fruit flies to find genes behind behaviour, and Ronald Konopka, a graduate student there, chose the circadian clock. He mutagenized flies and screened their offspring for altered rhythms in adult emergence from the pupal case and in locomotor activity. Three mutants stood out: one was arrhythmic, one ran with a period near 19 hours and one near 28 hours. Genetic mapping put all three on the X chromosome at what behaved as a single locus, which they named period. Konopka and Benzer published the result in PNAS in 1971. The gene itself was cloned in 1984 by two other groups, the laboratories of Jeffrey Hall and Michael Rosbash and of Michael Young. Their work on how period and its partners keep time earned those three the 2017 Nobel Prize. The screen made the clock a problem that genetics could take apart.

✓ verbatim Konopka & Benzer, Proc. Natl. Acad. Sci. USA 68:2112–2116 (1971), 'Clock mutants of Drosophila melanogaster'.
Friedrich Stephan

Friedrich Stephan

With Irving Zucker, showed in 1972 that bilateral lesions of the suprachiasmatic nucleus eliminated circadian rhythms of drinking and locomotor activity in rats.

By 1970 physiologists knew that mammals keep circadian rhythms in constant conditions, so something inside the animal had to keep time, and light through the eyes had to reset it. Where in the brain was the clock? Friedrich Stephan and Irving Zucker made electrolytic lesions in the hypothalamus of albino rats and recorded drinking and running over many days. Rats with bilateral lesions of the suprachiasmatic nucleus, a small paired cluster of neurons just above the optic chiasm, lost their circadian drinking and activity rhythms. Lesions of the medial preoptic area did not abolish the drinking rhythm. They published the work in PNAS in 1972. That same year Robert Moore and Victor Eichler reported that SCN lesions abolished the daily rhythm of adrenal corticosterone in rats. An electrolytic lesion destroys every cell and fibre in its path, so these studies showed that the region was needed, not how it kept time. They pointed every later search for the mammalian clock at the SCN.

✓ verbatim Stephan & Zucker, Proc. Natl. Acad. Sci. USA 69:1583–1586 (1972), 'Circadian rhythms in drinking behavior and locomotor activity of rats are eliminated by hypothalamic lesions'.
Martin Ralph

Martin Ralph

With Russell Foster, Fred Davis and Michael Menaker, transplanted SCN-region tissue into hamsters whose own SCN had been destroyed and found that the restored rhythm ran with the donor's period, not the host's.

A lesion shows that a region is needed, but the SCN might only relay a rhythm made elsewhere. Martin Ralph, working with Michael Menaker, had an unusual tool: a hamster carrying a mutation they named tau, reported in 1988, which shortened the free-running period. Hamsters with one copy ran fast and those with two ran faster still. That gave a label for whose clock was running. Ralph, Russell Foster, Fred Davis and Menaker destroyed the SCN of hosts, which became arrhythmic, and implanted SCN-region tissue from donors of a different genotype. In animals whose rhythms came back, the period matched the donor's genotype, whatever the host's own genotype was. They published the result in Science in 1990. The tissue transplanted was a region, not a single cell type, and the measure was locomotor activity, so the experiment did not show which cells keep time. It showed that the SCN sets the period of the rhythm it restores, which is what a pacemaker does.

✓ verbatim Ralph, Foster, Davis & Menaker, Science 247:975–978 (1990), 'Transplanted suprachiasmatic nucleus determines circadian period'; Ralph & Menaker, Science 241:1225–1227 (1988).
Emmanuel Mignot

Emmanuel Mignot

His group at Stanford used positional cloning to find that inherited narcolepsy in a dog model is caused by an autosomal recessive mutation in the hypocretin (orexin) receptor 2 gene.

Narcolepsy causes overwhelming daytime sleepiness and, in many patients, cataplexy: sudden loss of muscle tone, often set off by strong emotion, while the person stays conscious. Some dogs inherit a similar disorder, and Stanford researchers kept affected lines for study. Emmanuel Mignot's group mapped the canine gene by positional cloning, following the disorder through families until the region narrowed to one gene. In 1999 Lin and colleagues reported in Cell that canine narcolepsy in their model is caused by disruption of the hypocretin receptor 2 gene, Hcrtr2, inherited as an autosomal recessive trait. Later that year in the same journal, Chemelli and colleagues reported that mice lacking the orexin peptide showed a narcolepsy-like phenotype. Most human patients, however, carry no such receptor mutation. In 2000 Thannickal and colleagues, in a different laboratory, counted 85 to 95% fewer hypocretin neurons in four narcolepsy brains. A dog gene had pointed to a transmitter system that turned out to be lost in human patients by another route.

✓ verbatim Lin, Faraco, Li, Kadotani, Rogers, Lin, Qiu, de Jong, Nishino & Mignot, Cell 98:365–376 (1999); Thannickal et al., Neuron 27:469–474 (2000).
Test yourself
Question 1 of 8 Score 0 / 0

Which pathway carries light information from the retina directly to the suprachiasmatic nucleus?
Sources & Integrity

Every structure, slide, simulation, and clip on this page is listed below with the source it came from. None of the 3D structures is drawn by hand or by AI: each one loads by accession ID from RCSB PDB or PubChem. This panel is generated from the lesson's own data file, so what it lists is what the page loads.

Molecular structures live data

  • CLOCK-BMAL1 — the transcriptional activator of the clock loop — RCSB PDB 4F3L · coordinates CC0 · Mouse CLOCK and BMAL1 bHLH-PAS domains, X-ray crystal structure (Huang et al., Science, 2012)
  • Orexin receptor 2 — the receptor mutated in narcoleptic dogs — RCSB PDB 4S0V · coordinates CC0 · Human OX2 receptor bound to suvorexant, a dual orexin receptor antagonist, X-ray crystal structure (Yin et al., Nature, 2015)

Simulations

  • Free-running drift · Original interactive — Biochemistrypedia (Deppmann Lab).

Scientist profiles & quotes

  • Ronald Konopka — Konopka & Benzer, Proc. Natl. Acad. Sci. USA 68:2112–2116 (1971), 'Clock mutants of Drosophila melanogaster'.
  • Friedrich Stephan — Stephan & Zucker, Proc. Natl. Acad. Sci. USA 69:1583–1586 (1972), 'Circadian rhythms in drinking behavior and locomotor activity of rats are eliminated by hypothalamic lesions'.
  • Martin Ralph — Ralph, Foster, Davis & Menaker, Science 247:975–978 (1990), 'Transplanted suprachiasmatic nucleus determines circadian period'; Ralph & Menaker, Science 241:1225–1227 (1988).
  • Emmanuel Mignot — Lin, Faraco, Li, Kadotani, Rogers, Lin, Qiu, de Jong, Nishino & Mignot, Cell 98:365–376 (1999); Thannickal et al., Neuron 27:469–474 (2000).

Quotes are verbatim from the cited source. Profile text is original to this site and was checked against the sources cited.

Animated explainer

Light Resets a Slow Clock · narration: an AI narrator voice (Gemini, Charon) reading narration written from this lesson's text · animation: AI-generated in code (Manim) by Claude, a schematic with no drawn molecular structures, plots computed from their equations · captions from the narration script.

Images & figures

This lesson carries no raster figures. Every structure is rendered live from RCSB PDB or PubChem, and every plot is drawn from its equation in the browser.

integrity Machines check the mechanics. The build confirms that every local file exists, that every question's answer key points at a real choice, and that any AI-generated scientific figure names the person who checked it; a weekly job re-resolves every accession ID against its database. Questions are original; MCAT-style items are written for this site and are not real AAMC questions. Banned sources (the Tymoczko textbook, publisher decks, copyrighted figures) are not used.

Like I'm five

Inside your head is a tiny clock that keeps track of day and night, even in a room with no windows. Each morning, light in your eyes nudges the clock so it stays on time. Another part of your brain decides when you are awake and when you sleep, and it flips you one way or the other instead of leaving you halfway. Some people are missing a few cells that hold the awake switch steady, so they fall asleep when they do not want to.
High school

A small region of the hypothalamus, the suprachiasmatic nucleus, works as the body's master clock. In rats, destroying it ends daily rhythms of drinking and activity, and in hamsters, transplanting it brings rhythms back with the donor's timing. Inside cells, clock genes make proteins that eventually switch their own genes off, and the delay makes the cycle last about a day. Light reaches the clock through special light-sensing cells in the retina. Sleep itself has stages that can be read from brain waves, eye movements and muscle tone, and a group of hypothalamic neurons that make orexin helps keep waking stable.
College

The SCN is needed for overt circadian rhythms in rodents and sets their period, while peripheral tissues such as liver keep their own oscillators that the SCN synchronizes. In each cell, CLOCK-BMAL1 activates Per and Cry genes through E-box elements; PER and CRY proteins accumulate, enter the nucleus and repress CLOCK-BMAL1, and their degradation releases the brake. Melanopsin-containing retinal ganglion cells, with rods and cones, drive the SCN through the retinohypothalamic tract and induce Per1. Sleep is scored in 30-second epochs from EEG, EOG and EMG into N1, N2, N3 and REM. Its timing follows a homeostatic pressure built up during waking and a circadian signal, and a model of mutual inhibition between sleep-active preoptic GABA neurons and wake-promoting nuclei explains fast, stable transitions. Orexin neurons in the lateral hypothalamus excite those wake-promoting nuclei; their loss in human narcolepsy type 1 destabilizes waking.
Graduate

The core loop is a delayed negative feedback oscillator: the repressor (PER-CRY) is made from genes its own target activates, and transcription, translation, phosphorylation, nuclear entry and regulated degradation impose a lag of hours. With enough delay and a steep enough repression, such a loop oscillates rather than settling. In mice, PER2::LUCIFERASE tissues sustained more than 20 cycles in isolation, and SCN lesions desynchronized peripheral clocks instead of stopping them. Entrainment of mice to light failed only when melanopsin and rod-cone phototransduction were all removed, so the intrinsic and classical photoreceptors overlap in function. On the sleep side, the flip-flop model of mutual inhibition is a useful simplification: in mice, activating preoptic GABA neurons that project to the tuberomammillary nucleus promoted NREM sleep while broader activation of preoptic GABA neurons promoted waking (Chung and colleagues, 2017), so projection target together with transmitter, not region, defines a sleep-promoting population. For narcolepsy type 1, the human postmortem series (four cases, twelve controls) showed selective loss of hypocretin neurons with MCH neurons spared, and an orexin receptor 2 agonist improved sleepiness and cataplexy measures in a 2023 phase 2 trial that was stopped early for hepatic adverse events. An autoimmune attack on orexin neurons is hypothesized but not established. Open question: what triggers the selective loss of these neurons, and can its timing be caught before symptoms begin?
Three clocks, one gene

Konopka's screen turned up three mutant fly lines whose rhythms differed in opposite ways: one had no rhythm, one ran short at about 19 hours and one ran long at about 28 hours. If these had been three separate genes, the result would have said only that many genes matter. Instead the mapping placed all three at one spot on the X chromosome. One gene with alleles that could speed up, slow down or stop the clock suggested a component of the timekeeper itself, not a gene that merely permits rhythmic behaviour.

— Konopka & Benzer, Proc. Natl. Acad. Sci. USA 68:2112–2116 (1971).
The 25-hour day that was not

Older isolation studies, in which subjects often controlled their own room lights, gave human free-running periods near 25 hours, and that figure entered many textbooks. Light from lamps switched on at the subject's chosen times can itself shift the clock. Charles Czeisler's group used a forced desynchrony protocol in dim, controlled light, scheduling days far from 24 hours so the clock could not lock on. In healthy young and older adults the mean intrinsic period was 24.18 hours, and individuals varied around that value.

— Czeisler et al., Science 284:2177–2181 (1999).
A dog gene and a missing cell type

The canine narcolepsy gene turned out to encode a receptor, Hcrtr2, for a pair of hypothalamic peptides, the hypocretins (also called orexins). That suggested a direct test in people: are the cells that make the peptide still there? Thannickal and colleagues immunostained postmortem hypothalami from four narcolepsy cases and twelve controls. Cases had 85 to 95% fewer hypocretin neurons, while the neighbouring melanin-concentrating hormone neurons were not reduced. The same system was broken in dogs and people, by different routes.

— Lin et al., Cell 98:365–376 (1999); Thannickal et al., Neuron 27:469–474 (2000).

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