Sleep - Why is sleep a basic human need?

Sleep is a natural periodic state of rest for the mind and body, in which the eyes usually close and consciousness is completely or partially lost, so that there is a decrease in bodily movement and responsiveness to external stimuli. It is said that one third of an individual’s life time is spent on sleep and a normal person sleeps 6-10 hours per day, though there may be individual variations. Sleep is essential for the normal function of an individual.

The activity of the brain produces electrical waves which can be recorded using electrodes placed within the brain or over the scalp. Recording the activity of the brain with these is known as electroencehalography (EEG). The waves of the brain behave in four basic ranges of frequency with variations in the activity of the brain as illustrated in the figure. b waves are seen dominantly when the brain is alert and active (i.e. – while awake).With the onset of sleep, the brain waves tend to slow down.

There are two types of sleep:

1. REM sleep – Rapid Eye Movement Sleep

2. Non-REM sleep or Slow wave sleep

REM sleep is known as so because, during this sleep there is a rapid movement of eyes in addition to the relatively increased movements of the body. About 20% of an adult sleep is REM type. Dreaming usually happens in REM sleep and the dreams seen during this sleep can be recalled while awake. During REM sleep the heart rate, blood pressure and the respiratory rate becomes increased but irregular and the skeletal muscle tone becomes decreased (i.e. the muscles become relaxed). The metabolism of the brain is increased. Other features of REM sleep include penile erection and teeth grinding (bruxism). In REM sleep large amplitude slow waves are suddenly replaced by rapid, irregular fast activity (b waves) similar to alert, awake state in EEG. Thus the term paradoxical sleep is used to denote REM sleep.

REM sleep is brought about by noradrenaline (norepinephrine) secreting neurons in the locus ceruleus of the Pons (part of the brain stem).

Non- REM sleep accounts for 80% of the time spent on sleep. This is a state of rest and accordingly, the body movements, respiratory rate, heart rate and vascular tone all tend to decrease. There may be a 10-20% reduction of the blood pressure. The metabolic rate is also markedly reduced compared to the awaken state and there is specific slowing of the electrical activity of the brain as manifested by the Electroencephalogram (EEG). Dreams may also be seen in the Non-REM sleep but these dreams are usually not recalled later. Sleep walking (somnambulism), bed wetting (noctural enuresis) and nightmares occur usually in Non-REM sleep. Non-REM sleep can be classified further based on the electrical activity of the brain into four stages:

Stage I – a waves are replaced by rhythmic q waves, slowing seen

Stage II – appearance of vertex sharp waves and sleep spindles

Stage III – further slowing of waves with increasing in amplitude

Stage IV – very slow but large waves (δ waves) appear

Non-REM sleep is brought about by the serotonin secreting neurons in the raphe nucleus of the medulla oblongata (part of the brain stem).

During a night, a person undergoes different stages of sleep, which is collectively known as the sleep cycle. In a particular cycle, the activity of the brain tends to slow down gradually from the awaken state and the slow wave sleep progresses from stage I to stage IV gradually. This is seen to achieve the maximum depth within 1 hour from the onset of sleep. There after, the activity of the brain tends to increase gradually, with the stage of slow wave sleep drifting back towards stage I. An episode of REM sleep may occur when the sleep returns to stage I denoting the ending of a single sleep cycle. Following the REM sleep, the activity of the brain decreases again in as the previous cycle. Four to five such sleep cycles may occur in a sleep of 6 hours. In subsequent cycles of sleep after the first cycle, the interval between REM episodes decreases while length of REM episodes increases. The depth of the sleep cycle also tends to decrease with time. Several times the individual could awake momentarily though these might not be recalled.

Sleep-wake cycle is governed by a hormone known as melatonin secreted by a glad known as the pineal glad situated at the back of the brain. The pineal gland is signaled about the light-dark cycle of the surrounding by the nerve pathways coming from the eyes. Melatonin binds to major receptors on cells of suprachiasmatic nucleus of the anterior hypothalamus (a region in the brain) – the region that regulates the circadian clock, which regulates the sleep-wake cycle. Disturbances to this cycle cause a feeling of being not well. This is what is experienced as jet lag. It is always better if the sleep-wake cycle goes along with the light-dark cycle, as that is physiological and natural. Therefore, having the main continuous sleep of the day during night rather than the day is better for the health and well-being.

Sleep is requirement and a basic need of all individuals to maintain normal function while awake. Sleep is essential:

1. To repair the micro damages in structures of the body which happens during the day time

2. To form synapses (connections) between neurons (nerve cells) in the brain to lay down memories of things learned during the day (This happens mainly in the REM sleep. So, sleeping continuously for at least 6 hours is important to improve the memory)

3. For growth and development in children (Many hormones required for growth, such as growth hormone, are released in large amounts during sleep. This is important for growth and development. Lack of sleep during the growing age results in poor growth and short stature)

4. For the normal function of the body (Some hormones in the body, such as cortisol, are released cyclically. These cycles are based on the sleep-wake cycle)

Therefore, every individual should sleep for at least 4-6 hour per day to stay alert, active and function optimally during the day time. This minimal requirement may vary from person to person and is usually high in the extremes of age (i.e. very young and very old). Knowing the physiological basis of sleep may be helpful to understand the importance of sleep for our day-to-day life.