It’s important for you to understand some basic facts about sleep, including the stages and cycles involved. You probably know that there’s a stage of sleep (and also a successful rock band) called REM. There’s also a stage called non-REM. In healthy adults (ages 20 to 55), 20 to 25 percent of time asleep is spent in REM while 75 to 80 percent is spent in non-REM. Within non-REM, there are three stages (simply called 1, 2 and 3). During a typical night, you may complete four or five cycles, going through the four stages cyclically (from 1 through 3 followed by REM and then back to 1 again). As you’ll soon come to realize, it’s pretty amazing — and quite complicated — what happens once your head hits the pillow.
Stage 1: Drowsy Does It
The transition between wakefulness and sleep is stage 1 of non-REM sleep. For people who fall asleep without difficulty, stage 1 tends to be short — about five minutes or so — and is characterized as a drowsy, relaxed state between waking and sleeping. During this drifting-off period — as your breathing and heartbeat slow, your body temperature falls and your muscles relax — it’s easy to be aroused or awakened. Healthy sleepers tend to spend around 5 percent of the night in stage 1.
Stage 2: Are You Sleeping
When you reach stage 2 of non-REM sleep, you’re still in a light stage of sleep, and you stay there for about 10 to 15 minutes. Interestingly, many people with sleep problems perceive this stage as still being awake — even when they’re actually sleeping. Half the night is usually spent in stage 2.
Stage 3: And Good Night
Stage 3 of non-REM (previously divided into stages 3 and 4) is deep sleep or slow-wave sleep (SWS). Stage 3 is the deepest stage of sleep, and people tend to spend around 20 percent of the night in this stage. When you’ve reached this point, you’re sleeping so deeply you’re hard to waken. In fact, if you are awoken, you may feel that sluggish, confused, please-let-me-go-back-to-sleep feeling. Most of slow-wave or deep sleep happens in the first third of the night. Later, toward the early morning hours of sleep, you spend more time in REM.
REM: Dream Time
If you’ve ever watched someone in REM sleep, you understand why this type of sleep is called rapid eye movement. During REM, our eyes move quickly from side to side. During this period of sleep, our brains are very active, even though our muscles are relaxed. If you were to see brain wave patterns during REM, you might be surprised to realize that they resemble wakefulness, even though you’re sound asleep. In fact, apart from occasional muscle twitches, the body is essentially paralyzed.
During REM is when most dreaming occurs. It’s definitely when our most vivid dreaming happens, which makes it a very active stage of sleep. Because it’s active, if you wake from REM sleep, you feel more alert than when you wake from deep non-REM sleep.
Once you complete your first REM stage, you’ve made it through the first cycle of sleep. That first cycle usually lasts around 90 to 100 minutes. After that, each cycle gets a bit longer, as the REM stage grows in duration.