Have you ever stopped to consider that fully 1/3 of your life is spent asleep? Indeed, sleep takes up more time each day than any other activity. But what actually happens while we are asleep?
Most people have heard of REM and non-REM sleep. REM, or Rapid Eye Movement sleep is the period in which we dream. The rest of the sleep cycle is made up of non-REM sleep.
Non-REM sleep is divided into four basic stages. We cycle through these stages throughout the night, with each cycle culminating in a period of REM sleep. The cycle takes between 90 and 110 minutes, after which it repeats.
Early in the night our bodies are in need of deep sleep, which means that we spend a larger portion of our sleep cycle during the deep sleep phases. As the night progresses we become better rested and spend more time in REM sleep.
To understand the stages of sleep, it is important to be familiar with the ways in which scientists measure the sleep cycle. The first measurement is taken with an electroencephalogram, or EEG. The EEG measures the brain waves as you sleep. Each stage is characterized by a different pattern of brain waves. The next measurement is of the muscle tone, which is taken with an electromyogram, or EMG. This measures the effect that different sleep stages have on the muscles. Finally the movement of the eyes is measured with an electro-oculogram, or EOG.
When you are awake your brain is quite active. Your brain waves have a high frequency, meaning that on a graph they occur close together, and low amplitude, meaning that on a graph they are short, without large spikes. The brain waves do not follow a regular pattern, instead changing continually as you progress through your day. These brain waves are called Beta waves.
When you relax your brain waves become more regular. The amplitude goes up and the frequency slows. These brain waves are called Alpha waves.
The first stage of sleep occurs when you first go to bed. You enter a restful state in which you are mostly asleep, but are still easy to wake. Your eyes will move slowly as you drift in a semiconscious state. Sometimes sleepers will experience “sleep starts” where sudden contractions of the muscles, called hypnic myoclonia or myoclonic jerks, cause a feeling of falling. As you drift into sleep your brain waves slow even further than while relaxing, drifting into a slower frequency, higher amplitude wave called a Theta wave. Usually stage 1 sleep does not last very long.
Stage 2 sleep is deeper than stage 1. Your eyes stop moving and your brain waves slow down. Occasional bursts of fast brain activity called sleep spindles and periods of greater wave amplitude called K complexes are typical of stage 2 sleep. Most of the brain waves are Theta waves in stage 2 sleep. Like stage 1 sleep, stage 2 sleep does not last more than a few minutes.
As you drift deeper into sleep your brain will settle into a slow pattern with a high amplitude called Delta waves. This is the beginning of deep sleep. Your body is restful, your eyes are still, and you are deeply asleep. In stage 3 sleep just under 50% of your brain waves are Delta waves, with spikes of higher activity in between the quieter periods.
Much like stage 3, this stage is characterized by Delta wave brain activity. Over 50% of your brain waves are Delta waves, but occasional bursts of higher activity still occur. It is very difficult to rouse a person from stage 4 sleep. Interestingly, it is during stage 4 sleep that most cases of sleepwalking, night terrors, and even bedwetting occur. Stage 4 sleep lasts the longest in the early part of the night, but gradually decreases in length as the night progresses, until it nearly disappears by early morning.
After the body has cycled from the 1st to the 4th sleep stage it will reverse the pattern, returning to 1st stage sleep just before entering REM sleep. As you begin to dream there are many changes in your body. Your breathing becomes rapid and irregular, your eyes begin to move, often with a jerky motion, and your heart rate becomes elevated. At the same time your brain waves become active, much like those found while you are awake. The body produces a chemical that paralyses the muscles while you are dreaming, to keep you from thrashing about and hurting yourself. This is measured by the EMG, which picks up a sudden and drastic loss of muscle tone. If you are woken during REM sleep, you will likely remember your dream in full detail.
Most people will pass through this cycle of sleep stages 3-5 times throughout the night. Early stages will not have long periods of REM sleep, but as the night progresses your dream stage will lengthen. It has been theorized that we use REM sleep to process the information taken in through the day. Interestingly, infants spend 50% of their sleeping time in REM sleep. As we grow older, we require less REM sleep. Adults spend approximately 20% of the night in REM sleep.