Circadian Rhythms & Sleep

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.