States of Conciousness

History of Sleep Research
Sleep Disorders
Stages of Sleep
Nightly Cycle of Sleep and Dreams

Stages of sleep and Brain Waves

Beta waves
(very low amplitude, high frequency; 13 to 30 waves/sec)
A person is awake and active (in a state of alertness ). They are the fastest [EEG] waves and signal an active cortex and an intense state of attention. Irregular register (unsynchronized).
Alpha waves
(low amplitude, 8 to 13 waves/sec)
A person is awake and relaxed, with closed eyes. Neurons are firing at different times. Regular register (synchronized).
Theta waves (low-medium amplitude, spike-like waves; 3-7 waves/sec) A person is sleepy, already sleeping, or in a sleep transition. It can be observed in from the hippocampus. Theta rhythm is also observed in REM sleep. Because the hippocampus is involved in memory processing, the presence of theta rhythm during REM sleep in that region of the brain might be related to that [memory] activity.
Delta waves
(high amplitude, low frequency; 3 waves/sec)
A person is deep asleep*. Neurons, which are not engaged in the processing of information, are firing all at the same time. Therefore, the activity is synchronized. Waves are large and slow.
60 to 70
Maximal retraction of the pupil and nictating membrane follow the volleys of ocular movements

Stages of sleep through one night divided in cycles

A typical night of sleep consists of the repetition of a 90 to 110-minute cycle of REM and NREM (non-REM) sleep. The time spent in REM sleep is represented by a light-blue bar. The first REM period is usually short (5-10 min), but tends to lenghten in sucessive cycles. Conversely, stages 3 and 4, which together are often referred to as "delta sleep", dominate the slow-wave sleep periods in the first third of the night, but are often completely absent during the later, early morning cycles. The amount of stage 2 slow-wave sleep increases progressively until it completely occupies the slow waves period toward the end of the night. (Based on Kelly, 10).

The electroencephalograph (EEG) shows patterns of electrical activity during different stages of sleep. Brain waves of an awaken person and of those of a person in rapid eye movement (REM) sleep (when dreaming occurs) are similar in frequency and amplitude. In non-REM (NREM) sleep (stages 1, 2, 3, 4) brain waves have a higher amplitude and a lower frequency, indicating that neurons in the brain are firing more slowly and in a synchronized fashion.

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Author: Silvia Helena Cardoso, PhD. Psychobiologist, master and doctor in Sciences by the University of São Paulo and post doctoral fellowship by the University of California, Los Angeles.