Thanks to our natural internal rhythms, or biological clock, we tend to (or at least hope to!) fall asleep with the moon and wake up with the sun. What happens in between, however, is a bit more complicated.
Sleep happens in stages, and these stages are measured by brain wave activity, which cycles up and down throughout the night.
- Stage 1. As you begin to dose off, you drift from relaxation into stage1 sleep. This very beginning, and very light, stage of sleep is characterized by theta waves, which are very slow frequency brain waves. Theta waves are similar to what our brain waves look like when we are awake.
- Stage 2. Just minutes later, you enter Stage 2, where you are still in a light sleep characterized by theta waves, but every minute or so, the waves become more frequent. If you are woken up during either one of these theta stages, you might not even realize you had fallen asleep.
- Stage 3. About half an hour after you first fall asleep, you enter stage 3 and the beginning of a deeper level of sleep. This stage is characterized by delta waves, which are the slowest brain waves.
- Stage 4. Within about the first hour of sleep, you enter stage 4, the deepest level of delta sleep. If you are woken up during delta sleep, you are likely to feel groggy and disoriented.
Collectively, these four basic stages are known as non-REM (non-rapid-eye-movement) sleep and they represent about 80 percent of your entire night's sleep.
What About REM Sleep?
The REM (rapid eye movement) stage is the period of time during which you dream. REM sleep occurs periodically thoughout the night and accounts for the remaining 20 percent of time you spend sleeping. If you are woken up during REM sleep, you are most likely to have a clear memory of your dreams.
This is how it works: Your brain cycles up and down through the four basic stages of sleep about four or five times a night. So, in a normal night's sleep, you repeatedly drift into stage 1, move down through to stage 4, then back up through stages 3, 2 and 1 again. After the first stage, however, REM sleep takes the place of stage 1 sleep. Each full cycle generally lasts around 90 minutes so that you go into a dream state approximately every 90 minutes. As the cycle continues through the night, REM stages last longer and delta stages get shorter, until the morning, when there is no more delta stage and you wake up.
Sources:
Hall, R. "Psychology World: Stages of Sleep." Missouri University of Science and Technology. 1998. Web. 27 Aug 2010.
http://web.mst.edu/~psyworld/sleep_stages.htm
Harvard Medical School Division of Sleep Medicine. "Natural Patterns of Sleep." 18 Dec 2007. Web. 27 Aug 2010
http://healthysleep.med.harvard.edu/healthy/science/what/sleep-patterns-rem-nrem
University of Maryland Medical Center Sleep Disorders Center. "Normal Sleep." 3 Aug 2010. Web. 27 Aug 2010.
http://www.umm.edu/sleep/normal_sleep.htm
