Sleep timing is governed by two interacting systems: a homeostatic sleep pressure that builds up the longer you stay awake, and a circadian process that promotes wakefulness at certain times of day independent of how tired you are. The circadian alerting signal rises through the evening to counteract mounting sleep pressure, which is why you can feel a 'second wind' late at night despite having been awake all day.
A small cluster of cells in the hypothalamus, the suprachiasmatic nucleus, acts as the body's master circadian pacemaker, generating a rhythm close to but not exactly 24 hours. It is reset daily to the external day by light signals, keeping internal time aligned with the solar cycle.
Light is the primary signal that entrains the internal clock, detected not by the rod and cone cells used for vision but by a separate class of retinal cells most sensitive to blue light. Because of this, evening exposure to artificial and screen light can delay the clock and push sleep later.
Sleep is not uniform but moves through repeating cycles of roughly 90 minutes, alternating between non-REM stages (including deep slow-wave sleep) and REM sleep where most vivid dreaming occurs. Deep slow-wave sleep dominates early in the night while REM periods lengthen toward morning.
The authors argue that persistently curtailed sleep is associated with raised risks of metabolic problems, cardiovascular disease, impaired immune function and cognitive deficits, through pathways such as disrupted glucose regulation and hormonal balance. They present these as correlations and plausible mechanisms rather than claiming sleep loss is the sole cause of any one disease.
People differ systematically in their preferred sleep timing (morning 'larks' versus evening 'owls') largely because of genetically influenced differences in their internal clocks, not merely habit or discipline. Social schedules built around early starts therefore impose a biological disadvantage on late chronotypes, producing a mismatch the authors call social jetlag.