Contrary to popular belief, sleep is not simply a state when the brain is resting, but a dynamic, complicated condition when the brain is quite active. There are different stages of sleep. Each one is a bit different in terms of the type of brain activity occurring, the ease of awakening, and the presence of certain sleep disorders. Certain stages of sleep may be particularly important not only for maintaining alertness, but also for consolidating learning and memory.
What are the stages of sleep?
Sleep is not a static condition, but a complex series of sleep stages that repeats itself in a specific pattern over a night’s sleep. There are five distinct stages of sleep: stages 1, 2, 3, and 4 (all of which comprise “non-REM sleep”), and the fifth stage, REM (rapid eye movement) sleep.
- Each stage of sleep is defined by specific electrical patterns in the brain. While a person sleeps, it is possible to detect each of these sleep stages using a device which senses the electrical activity in the brain, called an electroencephalogram (EEG).
- There are also characteristic changes in muscle activity, eye movements, and breathing during the various stages of sleep.
- Measuring sleep and evaluating sleep disorders is performed using a technique called polysomnography. In this test, sensors are measure brain and muscle activity, eye movements, breathing, and sometimes other parameters such as oxygen in the blood, leg movements, and the cardiogram (EKG).
- During a normal night, the brain cycles through the different sleep stages from stage 1 to REM and then begins again with stage 1 (FIGURE 1). During some stages (particularly light non-REM sleep), the person can be awakened more easily, while during deep non-REM sleep it is particularly difficult to awaken.
Figure 1: Normal Sleep Structure in Young
Kales and Kales. N. Engl. J. Med.1974;209:487-499
Waking: When someone is awake, their brain is occupied with many functions.
- The patterns seen on an EEG during the waking period are ”chaotic”, meaning that different patterns are occurring independently.
- During waking, breathing is irregular and there is typically a lot of muscle and eye movement activity.
Stage 1: Stage 1 sleep could also be considered drowsiness.
- During this stage, a person’s eyes are closed, their breathing becomes more regular, and they have slower, rolling eye movements.
- They are also less aware of their surroundings than during the waking stage, but are easily aroused back to full wakefulness.
- This stage can occur when a person is quietly resting; “dropping off” briefly while watching television, reading, or being at an uninteresting lecture.
- The person may also have what are known as "hypnogogic experiences" - dream-like sensations of falling, hearing voices, or seeing flashes of pictures.
- Stage 1 may last for five to 10 minutes at the beginning of sleep, and accounts for a relatively small percentage of total sleep time (about 5%) in healthy adults.
Stage 2: During this phase, the person is even less aware of their surroundings, but is still easily arousable.
- During this stage, the person’s heart rate slows, breathing becomes even more regular, and their body temperature decreases. Muscles relax further.
- Brain activity shows some characteristic patterns, called sleep spindles and K-complexes, which are not seen during wakefulness or drowsiness.
Stages 3 and 4: These stages are deep sleep stages also referred to as “SLOW WAVE SLEEP”.
- Brain activity shows slower, more regular patterns. Muscles relax further, and breathing is very regular.
- During these stages, especially in stage 4, it is extremely difficult to arouse the sleeping person.
- Most people have had the experience of being awakened 30-60 minutes after falling asleep by the telephone, and finding themselves very confused for a few minutes or even not remembering the conversation the following day. This most likely occurs from being woken up from slow wave sleep.
REM Sleep: REM is very different from the other stages of sleep.
- Brain waves actually look more like the “chaotic” patterns seen in wakefulness than like other stages of sleep. In fact, the brain is very active during REM sleep, but the sleeping person is not aware of their surroundings.
- Most dreaming occurs during REM sleep. Although brief dream images can occur during other stages, the kind of complicated, bizarre, plot-driven experiences most people think of as dreams occur during this stage.
- Many researchers believe that REM sleep in particular is required for the consolidation of certian memories.
- During REM sleep, the body normally can't move, although brief twitching can occur. Yet the brain is very active with dreaming. Sometimes this lack of movement or paralysis can persist briefly after awakening, particularly in a person who is sleep deprived. This may be frightening but is completely normal.
- More prolonged sleep paralysis can occur in people with a neurological condition called narcolepsy.
What are circadian rhythms? Why are they important?
Our bodies have a built in alarm clock which tells us when to sleep and when to awake. This alarm clock is called the circadian rhythm. The circadian rhythm regulates not only sleep, but other processes such as body temperature and the secretion of certain hormones such as melatonin, cortisol, and growth hormone. The “biological clock” is located at the base of the brain, inside the hypothalamus, in an area called the suprachiasmatic nucleus (SCN).
In a healthy person, circadian rhythms follow a characteristic pattern over a 24-hour cycle (FIGURE 2).
- Melatonin, a hormone closely associated with sleep, is mainly secreted just after bedtime.
- Temperature tends to fall in the evening; it is this drop in body temperature that is partly responsible for the desire to sleep at that time. In the early morning, temperature begins to rise, signaling the body that it is time to awaken.
- Normally, this system is regulated by a number of external cues - daylight is one of the most important ones.
- When the system is disrupted (by travel across time zones or by working nights) the body will have conflicting signals for sleep and wakefulness.
- For example, when a person travels to Europe from the eastern United States there is a six-hour time difference. If the person arrives in Europe in the morning, their body will still be on Eastern U.S. time. Their body will think it is still night time. Their their body temperature will decrease, melatonin will be released and they will have a strong desire to sleep.
- Likewise, the opposite is true for night time such that their body will be six hours behind and still in waking mode. All of the physiological changes associated with sleep such as drop in body temperature and release of melatonin will not be present.
- These disruptions cause what is known as jet lag.
Figure 2: Melatonin concentrations in a normal man at baseline and after six nights of reversed light. From Brzezinski A, NEJM 1997;336:186-95.