Physiology Of Sleep

Success cannot be achieved at the expense of sleep. In this article we will discuss the definition, physiology, stages of the sleep cycle and its regulation.

Definition of sleep

Sleep is defined as an unconscious state from which a person can be aroused from, by sensory or other stimuli

Reticular Activating System

Before you dive deep into the physiology of sleep, you need to know about the Reticular Activating System (RAS). The Reticular Activating system is situated at the core of the brain stem, in the midbrain, pons, and medulla. It receives afferents from all the sensory pathways including visual and auditory pathways, and efferents from RAS activate the whole cerebral cortex via the thalamus. This activation is a non-specific activation of the whole cerebral cortex in contrast to the activation of specific areas of the cerebral cortex by individual sensory pathways. This non-specific activation is what keep the mind in alertness and conscious state

EEG

EEG stands for Electro Encephalogram. Simillar to an ECG, it is recorder by placing electrodes on the scalp. It records the synchronous firing of over 1000 neurons at a time making wave patterns. Several classes of wave patterns have been identified in the EEG.

Alpha Wave

Alpha wave is the predominant wave pattern. This occurs when the eyes are closed and the mind is wandering. This pattern occurs when awake, while being drowsy or non-vigilant. This wave has a frequency of 8-12 Hz and a wavelength of 50-100 μV.

Beta Wave

This occurs when the person mentally concentrates on something such as after a sensory stimulation. This occurs when eyes are open and the brain is receiving high levels of environmental stimulation. Higher frequency, low voltage waves, from 13-30 Hz fall into this category.

Gamma Wave

Gamma Oscillations are in frequencies ranging from 30-80 Hz. This occurs when an arouses person tries to focus attention. This is known as an ‘alerting response’

Theta Wave

This occurs in children and people with brain diseases in the wakeful stage. In healthy adults, this is seen during visual imagery,

Delta Wave

This wave pattern is only seen in the deeper stages of sleep in healthy adults. This is also seen physiologically in infants who are awake, and pathologically, in certain brain diseases.

Sleep Cycles and Stages of Sleep

Sleep is divided into sleep cycles. Each cycle in the physiology of sleep lasts 90 minutes, and around 5 cycles will occur in one night’s sleep. This 90-minute sleep cycle is divided into 2 main stages, the non rapid eye movement (Non REM) sleep that lasts 70 minutes. Rapid eye movement (REM) sleep that lasts 20 minutes.

There are many other feature to distinguish these stages other than the presence or absence of rapid eye movements.

REM Sleep

Mixed frequency eye movements are seen, but the tone of neck muscles is reduced. Voluntary muscle activity is inhibited. In REM sleep, the EEG wave pattern is similar to the alert state with eyes open. REM sleep is associated with dreaming, and teeth grinding (bruxism).

NREM Sleep

Non REM sleep is associated with reduced blood pressure, deep and infrequent breathing and reduced BMR. Sleep walking, night terrors and bet wetting are associated with non rem sleep, although dreaming is seen in REM sleep. Dreaming can also occur in the non REM sleep, but the dreams are far more vivid in the REM sleep.

Non REM sleep is further divided into 5 stages according to the depth of sleep which are identified by ECG change

Stage 0	Alpha waves	Eyes closed
Stage I	Alpha / Theta	Dozing
Stage II	Spindles / Theta	Sleep
Stage III	Delta / Theta	Deep sleep
Stage IV	Delta waves	Deep sleep

Waking

In the last two or three cycles, the stages 3 and 4 are successively reduced so that you fall less and less deep into sleep. REM sleep is incresed in the cycles prior to waking.