States of sleep

Two  sleep  states, non-rapid eye movement (NREM) sleep  and  rapid  eye  movement (REM) sleep, can  be distinguished on the  basis  of several physiological measures. Large numbers of cerebral cortical cells  fire  in  synchrony and  consequently their summed activity  produces potentials large  enough that they  can  be  recorded with  an  array  of scalp  electrodes as  electroencephalography (EEG). The EEG waveform varies in frequency and the frequency ranges are conventionally grouped: alpha (8–13 Hz), beta (13–30 Hz), delta (0.5–2 Hz), and theta (4–7 Hz). When awake, the EEG waveforms are of low amplitude and high (alpha) frequency in the Figure and are illustrate as desynchronized.  Non-REM (NREM) sleep has high-amplitude, low-frequency (synchronized) EEG waveforms.

         Figure  EEG waveforms recorded from the  human  brain when  awake  and  asleep.

During NREM sleep, muscle tone is retained and postural adjustments (turning over) are infrequently made. Heart rate, Respiration rate and mean arterial blood pressure all fall by gastrointestinal motility increases. Many growth hormone secretions occur in the duration of NREM sleep.

On falling asleep the EEG frequency progressively decreases, passing through four stages of NREM sleep. Stage 2 is interspersed by higher frequency bursts called sleep spindles and large spikes called K complexes. Sleep spindles may have a role in motor learning. K complexes are brief synchronized episodes of widespread cortical inactivity (cortical down time). Stages 3 and 4 are collectively referred to as slow-wave sleep because it is characterized by delta waves. In passing from stage 1 through to 4 it becomes increasingly difficult to arouse the sleeper. Brain scan shows that cerebral blood flow and glucose utilization fall by as much as 40% in NREM sleep.

In REM (paradoxical) sleep the EEG resembles in which of the aware state. Muscle tone is absent except for transient contractions of extraocular eye muscles (hence rapid eye movement sleep). Respiration rate, heart rate, mean arterial blood pressure, and core temperature become irregular. People aroused from REM sleep usually report that they were dreaming. At this time dream content is in short-term memory and is rapidly forgot- ten unless rehearsed. In NREM sleep Dreaming can also occur. In the REM sleep the brain is as metabolically active as it is when awake.

During a typical night’s sleep in figure adults drop rapidly into  deep NREM sleep and then REM and NREM sleep alternate about every 90 min with increasingly longer periods of REM sleep  as the  night progresses. After sleep deprivation an individual spends more time in NREM.
The proportion of time spent asleep changes dramatically during development. Human fetuses sleep (mostly REM) almost all the time. This falls to 17–18 hours sleep (50% REM) for babies born at term. The amount of time spent in stage 4 sleep falls exponentially with age. Between 10 and 70 years the proportion of REM sleep is constant at about 25% of total sleep time and declines in the elderly. Many vertebrates sleep but only homeotherms have REM sleep.

Figure:  Distribution of sleep stages during a typical night’s sleep. Dark bars are rapid eye movement sleep periods. NREM, Non-rapid eye movement sleep.