Most people know that when we sleep we pass through different phases - the 5 stages of sleep. Actually five different phases called stages 1-4 and REM (rapid eye movement) sleep. What some people don't know is that we don't pass from stage one through to REM sleep only once throughout the night. We go through this cycle repeatedly, with each cycle lasting from 90 to 110 minutes, and each time starting back again at stage 1. On average, we'll spend about 50% of our sleep time in stage 2 sleep, 20% in REM sleep and the other 30% in stages 1, 3 and 4 combined. Infants, however, will generally spend half their time asleep in REM.
Stage one can be defined as light sleep. It's where we're just starting to drift off to sleep but can be awakened fairly easily. If you are awakened during this stage, you will likely remember bits and pieces of visual imagery. It's in this stage that our muscle activity slows and our eyes start to slow down. During this stage some people experience what are called hypnic jerks or myoclonic twitches - a sudden movement similar to the little jump you make when startled or scared.
In stage two, our eye movement ceases and our brain waves slow down. In this stage, your brain will experience rapid waves that come in bursts which are referred to as sleep spindles. As we move into stage three, our brains go through smaller, faster waves and these are accompanied by considerably slower waves called delta waves. As we progress into stage four of sleep, our brains produce only delta waves and it's these last two stages combined (stage 3 and 4) that together are referred to as deep sleep.
During deep sleep there is no muscle activity or eye movement. It's in these two stages that some children may experience sleepwalking, bedwetting or night terrors. If awakened out of this deep sleep, you'll likely feel disoriented and groggy for the few minutes it takes you to adjust or clear the sleep from your head.
During the next stage of the 5 stages of sleep, REM sleep, our eyes move rapidly, jerking back and forth underneath our eyelids (hence the name Rapid Eye Movement) and we experience a temporary paralysis of our limb muscles. We also start to experience an increase in the pace of our breathing although it will be more irregular and shallow. It's also in this stage that dreaming begins accompanied by an increase in heart rate and a rise in blood pressure.
People will first enter REM about 70 to 90 minutes after initially falling asleep. During the first few hours of a person's sleep time, they'll experience less REM and more deep sleep. As they move into the middle hours of their sleep time, they'll experience more REM and less deep sleep and by the end of their sleep time, they'll spend the majority of their time in stages 1, 2 and REM sleep.
Since our bodies lose some of their ability to regulate our core temperature during REM sleep, hot or cold bedroom temperatures can more easily disrupt our sleep during this stage. If our bodies go through a disruption of REM sleep one night, the following night, our cycles of sleep will differ as we try to catch up on our missing REM sleep from the previous night.