To most people, sleep is just sleep. You go to bed, you sleep, and you wake up…and sleep is just one homogeneous state. However, as we learned in Sleep Step 2, this is not the case. There are actually two primary types of sleep, REM and non-REM, and non-REM has four stages of its own. Now it’s time to learn when we are in each stage throughout the night.
Our sleep takes on a sort of cycle throughout the night, alternating between the stages of non-REM and blocks of REM sleep. As stated earlier, about 25 percent of sleep is REM and 75 percent non-REM. However, these proportions are not equally distributed throughout the night. Non-REM sleep dominates the first half of the night, while three-fourths of a night’s REM sleep occurs in the last half of the night.
All very fascinating…but first, one has to fall asleep!
Sleep onset can be identified within a few seconds by studying the sleeper’s EEG levels. When we are awake with our eyes closed our EEG contains high levels of alpha waves. However, as soon as we fall asleep these alpha waves cease, a detectable change on the polygraph that shows almost exactly when the waking state ends and sleep begins.
Into non-REM Sleep – The First Sleep Cycle
After sleep onset the typical sleeper goes to stage 1 of non-REM sleep. It is rather easy to awaken somebody at this point in sleep onset stage 1. The sleeper gets more difficult to arouse when he or she next progresses to stage 2 of non-REM sleep, and a stimulus that will awaken a stage 1 sleeper often just triggers a K complex in stage 2 (you’ll remember from Sleep Step 2 that a K complex is a mountainous-looking recording on the EEG polygraph).
Then the sleeper progresses into stage 3, followed by stage 4 sleep. As you might recall, these stages are what is considered "deep sleep,” a sleeper is much harder to awaken at this point of the cycle. Stage 4 lasts for 20-40 minutes in this first cycle before things shift and the sleeper starts moving into lighter sleep.
The sleeper may go briefly back into stage 3 or straight to stage 2. They remain here for 5 to 10 minutes before exiting non-REM sleep for the first time in the night.
Into The First Bit Of REM Sleep
With the first non-REM cycle done, the sleeper enters REM sleep. The first REM period of the night is quite short, usually lasting between just 1 and 10 minutes.
Whereas slow wave deep sleep (non-REM stages 3 and 4) is highly prevalent at the beginning of sleep, REM periods are short in the beginning and longer as the night goes on. Besides the characteristics listed in Sleep Step 2, sleep researchers can tell when a sleeper is in REM sleep by of the presence of saw-tooth waves, unique patterns in the EEG.
When REM sleep ends and transitions again to non-REM sleep, the sleeper may make some body movements and have a very brief arousal. There is often a change in body position, like rolling over, but the arousals and movements are usually not remembered in the morning.
Through The Night
This non-REM to REM sleep alternation continues throughout the night, with varying properties for each cycle. The first sleep cycle, which we just completed above, usually lasts between 60 and 90 minutes. In the second cycle there is less stage 4 sleep and more stage 2 and REM sleep. The REM episode of the second sleep cycle lasts usually 10 to 20 minutes and the whole cycle lasts 100 to 110 minutes.
After the second cycle stage 3 and for sleep is virtually nonexistent. As a result, our deepest sleep tends to occur during the first third of the night. In contrast, REM sleep episodes get longer as the night goes on. Whereas the first REM period lasts only 1 to 10 minutes, the fourth or fifth period lasts 30 to 45 minutes. That’s enough time for some serious dreaming to occur!
Besides REM sleep, stage 2 sleep also dominates the last third of the night.
The sleeper continues to alternate from non-REM to REM, with the average cycle length being 90 minutes, until the journey through sleep is complete and he or she awakens to again accumulate more sleep debt so the journey can be begun anew the following night.