Physiology of SLEEP

How do we Measure Sleep?Electroencephalogram (EEG) measures the electrical changes in the brain. The electrodes are placed on the scalp. The wavy lines recorded by the EEG are called brain waves.

Electrooculogram (EOG) measures the electrical changes as the eyes rotate in its socket. The electrodes are placed either above and below the eye or left and right of the eye. 

Electromyogram (EMG) measures the electrical changes generated during muscle contraction. The electrodes are placed under the chin. 

EEG, EOG and EMG are recorded simultaneously and the patterns of activity in these three systems provide basic classification for the different types of sleep.  

Placement of electrodes to determine EEG,EOG and EMG

Brain, eye and muscle wave in NREM sleep have greater amplitudes and lower frequencies as compared to REM and wakefulness. The amplitude increases continuously, while their frequency decreases correspondingly from the time a normal person falls asleep, till he or she reach the deepest NREM sleep. 

Classification of Brain Waves

EEG associated with sleep from the highest to the lowest frequency: 

Beta waves (β) The frequency of beta waves range from 13-15 to 60 hertz (Hz) and an amplitude of 30 microvolt (μV). Beta waves are associated with wakefulness. 

Alpha waves (α) The frequency range from 8 to 12 Hz and an amplitude of 30 to 50 μV. These waves are found in people who have their eyes closed and relax or meditating.

Theta waves (θ) Frequency in the range of 3 to 8 Hz and amplitude of 50 to 100 μV. These waves are related with memory, emotions and activity in the limbic system. 

Delta waves (δ) It ranges from 0.5 to 4 Hz in frequency and amplitude of 100 to 200 μV. Scientists had observed delta waves in deep sleep and in coma patients because normal and healthy adults will not show large amount of delta waves. 

    Flat-line trace occurs when no brain waves are present and this is the clinical sign of brain death. 

STAGES OF SLEEP

Sleep is characterized by two distinct cycles, 

NREM sleep. Non-Rapid Eye Movement Sleep (NREM) is further classified into 4 stages: 

Rapid Eye Movement Sleep (REM) sleep

Stages 3 and 4 in humans are homologous to animal sleep stage of slow-wave sleep (SWS). A normal human sleep cycle starts with NREM stage 1, stage 2, stage 3, stage 4 and progresses to REM. This cycle is repeated several times throughout the night (between 4 to 5 cycles). The duration for each cycle has been identified ranging between 60 to 90 minutes. The next section explains the characteristics of each of the four stages in detail. 

Circadian Rhythms

Circadian Rhythm is one of the several intrinsic body rhythms modulated by the hypothalamus. The suprachiasmatic nucleus sets the body clock to 24 hours and is modulated by light exposure. The retino-hypothalamic tract allows light cues to directly influence the suprachiasmatic nucleus. 

Circadian rhythm allows the brain to regulate periods of rest during sleep (equivalent to battery re-charging) and periods of high activity during the wakefulness (equivalent to battery discharging). The nadir of the rhythm is in the early morning. The downswing in circadian rhythm after the apex in early evening is thought to initiate sleep and maintain sleep overnight for full restoration by preventing premature awakening. The morning upswing then facilitates awakening and acts as a counterbalance to the progressive discharge of wake neuronal activity, enabling cognitive function throughout wakefulness.