Stages and Cycles of sleep
5 stages of sleep
During stage 1, the brain begins to slow down to alpha and then theta waves and eye movements begin to slow. The amount of time in this stage varies, but it’s thought to be around 5-10 minutes. A person sleeping in stage 1 is easily awakened and many people experience sudden muscle contractions or a sensation of falling. A polysomnography typically finds a 50% reduction in activity between wakefulness and stage 1 sleep.
Still a relatively light sleep stage, stage 2 is characterized by sudden increases in brain wave frequency known as sleep spindles, also known as sigma bands or waves.
Sleep spindles are sudden bursts of oscillatory patterns that can be observed in EEG recordings. It’s important to note that sleep spindles are happen in differing magnitudes dependent on the region of the brain that they’re from. Fast spindles of 13-15 Hz happen in the centroparietal part of the brain, while the frontal brain produces slow spindles of 11-13 Hz.
Stage 2 is accompanied by several different physical changes: eye movement stops, body temperature begins to drop, and heart rate slows down. These changes are followed by a slowing down of brain waves. The brain waves that are recorded indicate spontaneous periods of muscle tone mixed with periods of muscle relaxation.
Another hallmark of stage 2 sleep are known as K-complexes, large waves that react to external stimuli while a person is sleeping. Abnormal K-complexes can be associated with less restful sleep, and even disorders such as epilepsy, restless legs syndrome, and obstructive sleep apnea.
Stage 3 sleep is characterized by exceptionally slow brain waves called delta waves that are interspersed with smaller, faster waves. This is colloquially referred to as deep sleep. The body repairs muscles and tissues, stimulates growth and development, boosts immune function, and builds up energy for the next day during stages 3 and 4. This is the stage of sleep that is associated with behaviors, referred to as parasomnias, like sleepwalking, night terrors, sleep-talking, and bedwetting. A person is difficult to awaken during this stage.
Deep sleep continues in stage 4 where the brain produces delta waves exclusively with none of the smaller faster waves characteristic of stage 3. When awakened from stage 4 sleep, people can feel disoriented for a few minutes. As we age, there is a natural decline in the time spent in these stages which may be due to a decline in need.
Average start time for a REM cycle is about 90 minutes into the sleep process, and each REM sleep stage can last up to an hour. The first stage of REM sleep is typically short, around 10 minutes, but the length increases as the night progresses. The average adult has 5-6 stages of REM each night. Brain waves mimic activity during the waking state. The eyes remain closed by move rapidly from side-to-side as dreams occur. Physical changes in the body can include heart rate and blood pressure increases and fast, shallow, irregular breathing patterns accompanied by varying degrees of muscle movements and twitching. Despite this twitching, paralysis of voluntary muscle groups also occurs. This phase of sleep is associated most with learning and memory, as the brain processes and consolidates the information from the day before so it can be stored in long term memory.
Cycles of Sleep
One sleep cycle refers to the completion of Stages 1-4 and REM. An adult will go through through anywhere from 3-6 cycles of sleep through the night dependent on many different factors. It’s important to note that as we age, the sleep cycle pattern evolves and we have more regular sleep patterns. Infants, children, and even young adults can have widely varied sleep cycles. For example, the percentage of REM sleep is highest during infancy and early childhood. As we get older, the percentage decreases. As we get even older, people tend to enter REM more quickly and remain in REM longer.