Around the world, and throughout the animal kingdom, sleep is a normal phase of each 24-hour period.
As humans, we are naturally diurnal, being awake during the day and sleeping at night. In fact, light and darkness play a large role in regulating the timing of our wakefulness and sleep. We are designed, under normal circumstances, to sleep when it becomes dark and to be awake when it is light.
Light perceived by our eyes signals a small part of our brain called the suprachiasmatic nucleus, which is the main pacemaker of our body clock. In this way, the light-dark cycle of our surroundings has a strong effect on the timing of sleep and wakefulness during each 24-hour period. This is called the circadian process of sleep regulation. The same phenomenon regulates our body temperature rhythm and many other internal biological rhythms.
Sleep is also regulated by the amount of time we have been awake. Broadly speaking, the longer we are awake, the more our sleep drive builds up. This is called the homeostatic process of sleep regulation. Together, the circadian and homeostatic processes help us to have good sleep when they operate without interference. This is called the two-process model of sleep regulation and was first proposed by Dr. Alexander Borbély.
Two States of Sleep
There are two distinct states of sleep: non-rapid eye movement (NREM) and rapid eye movement (REM). All stages of sleep contribute to the restoration of our body and mind.
Human NREM sleep is normally divided into 3 stages, based on the depth of sleep.
NREM1 (N1) is a transition stage between wake and sleep.
NREM2 (N2) is light sleep.
NREM3 (N3) is deep sleep.
Stage REM has many unique features. Most of our dreams occur in this stage. As opposed to the NREM stages, when our eyes have slow rolling movements, in REM sleep, our eyes move quickly as they track dream imagery. Our brain activity, as measured by electrodes on the scalp, is quite similar to waking activity. Our postural muscles are very relaxed; in fact, they are so relaxed we are unable to move. This temporary paralysis prevents us from acting out our dreams!
The NREM stages normally occur first at night, followed by an episode of REM. This series of stages, called a “sleep cycle”, is repeated 3-5 times during the night. As the night proceeds, the duration of N3 decreases from cycle to cycle, whereas the duration of REM increases. As a result, most of the deep NREM sleep occurs in the first third of the night, and most of REM in the last third.
For adults, this is what it’s like to have healthy, normal sleep:
- You feel rested during the daytime
- It takes about 10-20 minutes to fall asleep
- You sleep for 7-9 hours
- You might wake up briefly (1-10 minutes per awakening) during the night 1-4 times
- You might take an afternoon nap (up to 30 minutes), which is normal and healthy for some adults.