Sleeping states
Not all sleep is created equal.
Our 24-hour internal body clock is regulated by two systems: circadian and homeostatic. The circadian system is facilitated by melatonin, which is produced in darkness and makes us sleepy. The homeostatic system is responsible for balancing things out, bringing rest to a system that is not at rest. The chemical adenosine mediates the homeostatic system, inducing sleepiness as the day progresses (caffeine blocks this chemical; exercise boosts it).
On the vigilance end of our constant seesaw between sleep and wakefulness, our body releases dopamine, which is crucial for arousal, memory and motor function; histamine (which explains why antihistamines make us sleepy); and orexin, a neuropeptide lacking in those suffering from narcolepsy.
Supported by this daily roster of hormones and neurotransmitters, our body at night is ready to sleep. In The Sleep Solution, Chris Winter describes four levels of sleep. There are two stages of light sleep: these are the necessary passageway between wakefulness and sleep, taking up about half of our nighttime sleep.
From there we go into deep sleep or slow-wave sleep, the restorative state that enables the body to block out external noises and movement and helps us maintain sleep (it is also the time of greatest hormone production). Deep sleep makes up 10 -20% of a night’s sleep.
REM (rapid eye movement) sleep comes in four to five cycles of about 20-40 minutes, usually beginning an hour and a half after falling asleep.
While new theories suggest we do dream during deep sleep, REM sleep, which makes up about a quarter of our sleep, is generally known as dream sleep.