Intro | Stage 1 | Stage 2 | Stage 3 | Stage 4 | Wakefulness

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The EEG of a human during Wakefulness is characterized predominately by high frequency, low voltage brain wave activity (beta band, 13-24 cps). Increased beta wave activity is associated with alert problem solving and behavior.

When one begins to fall asleep, the EEG begins to slow. Brain wave activity in the alpha band is common. It is easy to be aroused by external stimuli. Stage 1 Sleep follows this transition from wakefulness, and is of brief duration (5-10 minutes). Stage 1 sleep is characterized by alpha (8-12 cps) and emerging theta wave activity (4-7 cps). During this stage, breathing and heart rate decline along with muscle tone and temperature.

During Stage 2 Sleep, breathing, heart rate, muscle tone, and temperature continue to slow. Brief bursts of high-frequency brain wave activity are superimposed on an EEG of varying frequency. These bursts of high-frequency activity are called sleep spindles.

As one moves deeper into sleep, physiological as well as EEG activity slows further. The slow-wave sleep of Stage 3 Sleep and Stage 4 Sleep is characterized by ever increasing amounts of delta wave activity (less than 4 cps). One usually enters slow-wave sleep toward the end of the first hour of sleep and stays in that state for approximately 30 minutes.

After stage 4 of the slow-wave period, the cycle reverses itself and slowly moves backward into lighter stages of sleep. This is when REM sleep, or the 5th stage of sleep occurs. REM stands for rapid eye movements, which are prevalent during this stage of sleep. Irregular breathing and heart rate are also characteristic of REM sleep. EEG activity during this stage is similar to that found during wakefulness, dominated by low-amplitude (voltage), high-frequency beta waves. Most of the dreaming we experience occurs during REM sleep. In a classic study, subjects who were awakened during REM sleep reported dreaming 78% of the time. When subjects were awakened during any of the other stages of sleep (Non-REM Sleep, NREM), they reported dreams only 14% of the time. Although the mind is very aroused and active during REM sleep, it is very difficult to awaken one while in this state.

During a night's sleep, we cycle through the various stages approximately four times. The first REM period is brief (only a few minutes long), but each successive cycle results in a REM period of greater duration. Shortly before we awaken, the typical REM period will last approximately one hour. As the duration of REM sleep increases throughout the night, the duration of NREM sleep decreases. This means that most slow-wave sleep occurs early in the night cycle, and most dreaming occurs later. Young adults will typically spend approximately 60% of their sleep cycle in light sleep (stages 1 and 2), 20% in slow-wave sleep (stages 3 and 4), and the remaining 20% in REM sleep. Infants and young children sleep longer, and spend a greater amount of time in REM sleep. With increasing age, the proportion of time spent in slow-wave sleep decreases, and there is a shift toward lighter sleeping. Cultural customs may affect sleep behaviors such a co-sleeping (parents and children sleeping together) or napping ("siesta cultures"), but do not affect the psychological or physiological experience of sleeping.


As demonstrated by the experiments that address the effects of sleep deprivation, sleep is a regulated process. The amount of sleep we experience is monitored by a physiological mechanism that then exerts control to ensure our survival and optimal functioning. Indeed, massive research efforts have sought an understanding of this mechanism. This research indicates that both the stages of sleep and varying levels of awareness while awake are modulated by the same neural mechanisms. The reticular formation responds to ascending sensory input by activating and thereby arousing the cerebral cortex. It does so via two pathways. The dorsal pathway projects to the medial and intralaminar nuclei of the thalamus, and then on to the cerebral cortex. The ventral pathway supplies information to the lateral hypothalamus, basal ganglia, and basal forebrain region. The basal forebrain region, in turn projects to the hippocampus (see Figures 7 and 8) and extensively throughout the cerebral cortex.

To date, three separate neurotransmitter systems have been identified that modulate arousal and wakefulness. The first of these to be discovered is the noradrenergic projects of the locus coeruleus (LC), which is located in the dorsal pons. The noradrenergic neurons of LC project diffusely throughout the neocortex, cerebellar cortex, hippocampus, thalamus, pons, and medulla. LC neurons show an abrupt increase in discharge rate upon wakening and are relatively inactive just before and during sleep. LC neurons are also more active during tasks that involve vigilant attention to external stimuli and when sudden and novel (and often stressful !), attention-grabbing stimuli appear. LC neurons also play help to inhibit REM sleep during wakefulness and slow-wave sleep. The second neurotransmitter system to be studied extensively is the serotongergic (5-HT) neurons of the dorsal raphe nuclei. These nuclei are also found throughout the reticular formation region of the medulla and pons, and like LC, they project diffusely throughout the brain including neocortex, thalamus, hypothalamus, basal ganglia, and hippocampus. Increased discharge of 5-HT neurons is associated with increased locomotion and EEG activation in cerebral cortex. Unlike LC noradrenergic neurons, however, 5-HT neurons show decreased activity with stress-inducing events. Serotonergic neurons of the raphe nuclei show their increased discharge rates with continuous, automatic movements such as grooming, pacing, and chewing. Similar to LC noradrenergic neurons, 5-HT neurons are most active during wakefulness. Their discharge rates are very low during slow-wave sleep and negligible during REM sleep. Acetylcholine is the third neurotransmitter associated with arousal mechanisms. There are two groups of acetylcholinergic neurons; one is located in the pons and the other in basal forebrain (the region just rostral to or above the hypothalamus). The evidence indicates that acetylcholine increases EEG signs of cortical arousal, alertness, and behavioral activity. This activity is mediated via both the pontine and basal forebrain nuclei.

Paradoxically, very early studies showed that damage to the basal forebrain region results in complete insomnia. Stimulation of the same area induces acute onset drowsiness and slowing of the EEG. Presumably, the areas responsible for these effects are not the same as the acetylcholinergic neurons also located in the basal forebrain region, as these neurons affect arousal in the opposite direction. Recent research has linked the slow-wave sleep inducing mechanisms within basal forebrain to neurons that regulate body temperature. This region, which is often referred to as the POAH, includes part of the basal forebrain, preoptic area, and adjacent anterior hypothalamus.

In summary, a review of research on the neural mechanisms of arousal control suggests the following relationships: 1) In general, nuclei within the pontine reticular formation stimulate acetylcholinergic neurons in the basal forebrain area, that in turn excite cortical neurons. Two neurochemical systems within the pontine reticular formation enhance cortical arousal under distinct and opposite behavioral conditions, one supporting arousal to external events the other supporting arousal to internal events. 2) Serotonergic neurons of the raphe nuclei of reticular formation stimulate acetylcholinergic neurons in support of automatic behaviors such as grooming and eating (internal arousal). 3) Noradrenergic neurons of the locus coeruleus of reticular formation stimulate the acetylcholinergic neurons in support of orienting responses made to novel events of possible threat (external arousal). 4) Thermoreceptors in POAH stimulate the sleep-promoting region in POAH in response to increased body temperature. 5) The sleep-promoting region in POAH inhibits the excitatory effects of acetylcholinergic neurons in basal forebrain and, in addition, directly inhibits cortical arousal. 6) Thermoreceptors in POAH also inhibit the acetylcholinerigc neurons in basal forebrain in response to increased body temperature.