The Sleep Cycle

Understanding the Five Stages of Sleep

Jan 31, 2010 Jessica Gore

The sleep cycle typically involves the regular progression through five phases. Using an EEG, sleep researchers have identified brain waves that accompany each stage.

Sleep research has identified, through the use of electroencephalogram (EEG,) five stages of sleep and two stages of wakefulness. These sleep stages represent progressively deeper sleep for stages one to four, with stage four being the deepest. The fifth stage, rapid eye movement (REM) sleep, is what is known as an active stage, and it is in this stage that dreaming occurs. Each stage of sleep or wakefulness is characterized by brain wave patterns recorded by the EEG.

Before the Sleep Cycle: Wakefulness

Wakefulness has two distinct phases, and most people can identify them without ever laying eyes on an EEG machine. Generally speaking, when people are awake, they are usually either active and alert, or relaxed and drowsy. For most people, these sleep cycles occur according to a fairly regular 24-hour biological clock.

Not surprisingly, there are correlating brain waves associated with each of these two states. When alert and concentrating, the brain exhibits waves on the EEG known as beta waves. Beta waves are typically the highest in frequency and the lowest in amplitude, and tend to not occur in any sort of consistent pattern, as the brain is usually responding to a multitude of sensory stimuli.

In a relaxed state, the brain waves slow down and become rhythmic. This is the state of mind experienced when almost-dozing in front of the television, or gazing absently out the window. Lower in frequency but higher in amplitude than beta waves, the brain waves associated with drowsiness are termed alpha waves.

Sleep Stage 1: The Onset of Theta Waves

As most will be aware, there is often a good degree of overlap between the states of restful drowsiness and early sleep. This overlap can be seen on the EEG as well. The shift from alpha waves to theta waves – the brain waves associated with early sleep – is a gradual one. Stage one sleep is characteristically light, and generally lasts about ten minutes.

Sleep Stage 2: Theta Waves With Sleep Spindles

During this sleep stage, the theta waves of stage one continue, but are interrupted by sleep spindles. The defining characteristic of sleep stage two, sleep spindles are short bursts of increased frequency in the brain wave pattern. Both stage one and two are considered to be light sleep. During these stages, the sleeping person is easily roused, and once awake, he or she might not be aware of having slept.

Sleep Stages 3 and 4: Delta Sleep

Difficult to distinguish, stages three and four are marked by the onset of delta waves and are often referred to collectively as delta sleep. This is the deepest phase of sleep, in which rousing the sleeping person is most difficult. Lowest in frequency and highest in amplitude, delta waves are least similar to waking brain activity. If roused during delta sleep, most people are disoriented and confused, and most likely to fall back into sleep.

REM Sleep: The End of the Sleep Cycle

REM sleep, the fifth stage of sleep, is probably the most well known stage of the sleep cycle. Once finished stage four sleep, the sleeping person gradually returns to near-wakefulness, however, instead of waking up or beginning again at stage one, he or she moves to active sleep.

During REM sleep, brain wave patterns are similar to those seen in relaxed wakefulness. REM sleep includes most of the dreaming that takes place during the night, and is thought to play a role in memory and in the brain development of children. Babies and young children spend significantly more of their sleeping hours in REM sleep than adults.

The Sleep Cycle During the Night

Every 90 to 100 minutes during the night, the brain completes one sleep cycle. During the first half of the night, the largest portion of the sleep cycle is spent in deep sleep. As the night progresses, an increasing proportion of sleep is spent in the REM stage. As a result, it is during the second half of the night that the majority of dreaming takes place. Over the course of a typical night in a healthy adult, about 60% of sleep is spent in light sleep, 20% is spent in deep sleep, and another 20% is devoted to REM sleep.


Santrock, J.W. and Mitterer, J.O. (2006) Psychology. Toronto: McGraw-Hill Ryerson.

The copyright of the article The Sleep Cycle in General Medicine is owned by Jessica Gore. Permission to republish The Sleep Cycle in print or online must be granted by the author in writing.
A Baby's Sleep Cycle Has More REM Sleep, Peasap A Baby's Sleep Cycle Has More REM Sleep
Typical Sleep Study Room, Romulusnr Typical Sleep Study Room
Stage One of the Sleep Cycle on an EEG, S. Jähnichen Stage One of the Sleep Cycle on an EEG
Stage Two of the Sleep Cycle on EEG, S. Jähnichen Stage Two of the Sleep Cycle on EEG
Stage Four of the Sleep Cycle on EEG, S. Jähnichen Stage Four of the Sleep Cycle on EEG
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