Circadian Rhythms & Sleep

The suprachiasmatic nucleus (SCN) is a paired cluster of approximately 20,000 neurons in the anterior hypothalamus, directly above the optic chiasm. It is called the master clock because it coordinates all peripheral circadian clocks throughout the body — in the liver, heart, skin, and virtually every other tissue — setting them in synchrony with the external light–dark cycle. The SCN receives direct photic input from melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) via the retinohypothalamic tract, allowing daylight to reset ("entrain") the clock. Even isolated SCN neurons in culture maintain rhythmic firing with a roughly 24-hour period indefinitely — demonstrating the cell-autonomous nature of the clock. The molecular mechanism of the circadian clock — the CLK/CYC–PER/TIM feedback loop discovered in Drosophila, which maps onto the mammalian CLOCK/BMAL1/PER/CRY system — was elucidated by Jeffrey Hall, Michael Rosbash, and Michael Young, who received the 2017 Nobel Prize in Physiology or Medicine for this work.

Melatonin is synthesised from serotonin in the pineal gland and released into the bloodstream under SCN control — specifically, melatonin is secreted during darkness and suppressed by light. Its rise in the evening signals to the brain and body that night has begun, coordinating the timing (but not the depth) of sleep onset. Melatonin does not directly induce sedation; it is a chronobiotic — it shifts the phase of the circadian clock. Supplement evidence: low doses (0.5–1 mg) timed to the desired sleep phase are effective for treating circadian phase disorders (delayed sleep phase, jet lag, shift work) and are first-line treatment for these conditions. For ordinary insomnia or sleep quality, the evidence is modest — melatonin shortens sleep onset by ~7 minutes in meta-analyses but does not reliably improve sleep duration or quality. Most over-the-counter doses (3–10 mg) are pharmacological rather than physiological, and chronic high-dose use can desensitise receptors. Short-wavelength (blue) light suppresses melatonin most potently; evening light exposure from screens delays its onset.

Social jetlag (Wittmann et al., 2006) is the chronic discrepancy between a person's biological clock and their social clock — the difference in sleep timing between work/school days and free days. A night owl forced to rise at 6 a.m. for work but sleeping until 9 a.m. on weekends has a social jetlag of approximately 3 hours — equivalent to weekly transatlantic jet lag. It is driven by the mismatch between individual chronotype (partly genetic, heavily influenced by age) and fixed social schedules that favour early morningness. Consequences: impaired sleep quality, reduced total sleep, daytime sleepiness, poorer academic and occupational performance, increased risk of depression, obesity (via circadian disruption of metabolic hormones), smoking, and higher alcohol consumption. Adolescents are most severely affected: chronotype shifts strongly toward eveningness in puberty (peaking around age 20), yet school start times typically remain fixed or even earlier. Delaying school start times by 45–60 minutes has shown improvements in attendance, academic performance, and mental health in controlled studies.

The American Academy of Sleep Medicine and Sleep Research Society recommend 7–9 hours per night for adults. Large-scale epidemiological studies consistently show J-shaped mortality curves — both short sleep (<6 h) and very long sleep (>9 h) are associated with increased mortality and morbidity, with the nadir around 7–8 hours. Importantly, most people cannot reliably self-assess whether they are getting sufficient sleep: after several weeks of 6 hours per night, cognitive performance degrades to the level of 24-hour total sleep deprivation, yet people rate themselves as "slightly sleepy" — not impaired.

Short-wavelength (blue, ~480 nm) light is the most potent activator of melanopsin-containing ipRGCs — the retinal cells that signal light to the SCN. These cells are exquisitely sensitive at night, when the circadian system expects darkness. Evening exposure to blue-light-emitting screens (phones, tablets, LED screens) suppresses melatonin secretion, delays the circadian clock, reduces total sleep time, and reduces slow-wave sleep. The effect is strongest in the 2–3 hours before habitual sleep onset. Blue-light-filtering screen settings or glasses partially (but not completely) mitigate this; the most effective approach is reducing overall light exposure in the evening.

Sleep deprivation is insufficient sleep opportunity — the person does not get enough time in bed to sleep adequately (due to work, social obligations, environment). Given sufficient time, they fall asleep rapidly. Insomnia disorder is defined by difficulty falling asleep, staying asleep, or waking too early despite adequate opportunity and circumstances — causing daytime distress or impairment. Ironically, insomnia patients often show hyperarousal (elevated body temperature, heart rate, metabolic rate, and cortisol) even at night; their problem is difficulty in achieving sleep despite wanting it, not simply short sleep. Treatment differs accordingly: sleep deprivation needs more time in bed; insomnia needs CBT-I, which typically includes a period of sleep restriction to consolidate sleep.

During N3 (slow-wave sleep, SWS), the cortex generates large-amplitude, low-frequency delta oscillations (~0.5–4 Hz). The hippocampus simultaneously generates sharp-wave ripples — bursts of activity that replay waking experiences. These hippocampal ripples co-occur with cortical slow oscillations and thalamic sleep spindles in a three-way coupling that facilitates the transfer of newly encoded memories from hippocampal short-term storage to distributed neocortical long-term storage (systems consolidation). The pituitary also releases the largest growth hormone pulse of the day during early SWS. SWS is the most homeostatic stage: sleep pressure is highest at the start of the night, SWS occupies early cycles, and its loss cannot be easily replaced by extra lighter sleep.