22. There are five stages of sleep that we cycle through each night.
There are two types of sleep: rapid eye movement, REM, and non-rapid eye movement, or non-REM sleep. Non-REM sleep consists of multiple stages, whereas REM sleep is just a single stage. Stage one is a non-REM stage that occurs when you first begin to fall asleep and generally only lasts a few minutes. During this stage, your heart rate and breathing slow down, your muscles start to relax, and you produce alpha and theta brain waves. The second stage of non-REM sleep is light sleep that happens right before you enter deep sleep. On average, it lasts for roughly 25 minutes.
Your heart rate and breathing will continue to slow down further. There are no eye movements, and your body temperature begins to drop. Besides, your brain waves spike up and down. Stages three and four are the final stages of non-REM sleep that are the deepest sleep stages. During these stages, your body performs various health-promoting tasks, including tissue repair and growth, cell regeneration, and strengthening your immune system. Approximately 90 minutes after falling asleep, you enter into stage five, which includes REM sleep. That is the primary dreaming stage of sleep. During phase five, your brain activity increases exponentially. When you fall asleep at night, you cycle through all of these stages multiple times.