The body regulates sleep through the circadian clock and sleep-wake homeostasis, which work together to determine sleep needs. The circadian clock, influenced by melatonin and core body temperature, signals when to sleep, while adenosine levels increase throughout the day, promoting sleepiness. Homeostatic sleep propensity balances the need for sleep based on elapsed time since the last adequate sleep episode. Observations from studies on cavers indicate that without external light cues, individuals can develop longer internal cycles, suggesting adaptability in sleep patterns.
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Sleep timing is controlled by the circadian clock, sleep-wake homeostasis, and in humans, within certain bounds, willed behavior. The circadian clock—an inner timekeeping, temperature-fluctuating, enzyme-controlling device—works in tandem with adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness[citation needed]. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone melatonin and a gradual decrease in core body temperature. The timing is affected by one's chronotype. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode.[17]
Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep.[18] Along with corresponding messages from the circadian clock, this tells the body it needs to sleep.[19] Sleep offset (awakening) is primarily determined by circadian rhythm. A person who regularly awakens at an early hour will generally not be able to sleep much later than his or her normal waking time, even if moderately sleep-deprived.
I'd heard that results like this are bunk, even though they are widely reported, and are the result of faulty experimental conditions. I'll see if I can find more about that.NeuronsAtWork said:I watched an interesting show on this subject a few years back. It followed a number of 'cavers'--scientists and geologists who would spend weeks underground exploring and mapping miles-long cave systems. They obviously had no conventional cycles of light and dark to influence waking and sleep patterns, and just allowed their bodies to determine the proper amount of sleep as needed. It turned out that, after a number of days, their internal 'day' and 'night' periods became progressively longer, so that they were eventually living on something like a 25 or 26 hour cycle. I don't know how that correlates to anything evolutionary, since it's my understanding that days were actually shorter in the past than they are now. Still, it's an interesting observation.
enosis_ said:A bit off-topic regarding sleep, but shows an effect when time is uncertain.