Let's Know about Sleep

Let’s Know about Sleep

Whenever the person works he gets tired and fatigue arises, which he reduces by sleep and rest. The RAS or Reticular Formation in brain conducts the sleeping and waking function.

Sleep is not simply non-waking but rather the interlocking of elaborative cyclical process, an alternation of different stages. Sleep seems to be characterized by the absence of behavior; it is a period of inactivity with raised threshold to arousal by external stimuli.

Stages of Sleep

During sleep, we usually pass through five sleep phases: Stage I, II, III, IV, and REM sleep. These stages progress in a cycle from Stage I to REM sleep, then the cycle starts over again with Stage I. We spend almost 50 % of our total sleep time in Stage II sleep, about 20 % in REM sleep and remaining 30 % in the other stages. Infants, by contrast, spend about half of their sleep time in REM sleep.

Fully Awake/Active Person: The pattern of gross electrical activity appears as a desynchronized mixture of many frequencies dominated by Beta waves of relatively fast frequency and low amplitude (voltage).

Relaxed: With relaxation and closing of eyes a distinctive rhythm appears, consisting of a regular oscillation at a frequency known as the alpha rhythm. In other words, in relaxed wakefulness, the record is likely to consist mainly of synchronized alpha waves of moderate voltage. The appearance of regular alpha waves in the record is a good sign the person is awake.

I. Drowsiness/Somnolence: As the person becomes drowsy, the amplitude of the alpha waves is reduced and at the some point it disappears and is replaced by much smaller voltage (slow waves) events of regular frequencies, known as theta waves, are seen.

This stage is called Stage-I Sleep, is a period of transition from relatively unsynchronized beta & gamma brain waves to more synchronized but slower alpha waves and then to theta waves. It is difficult to pinpoint the actual point of sleep. During this period there is a slowing of heart rate and a reduction of muscle tension. Many subjects awakened during this stage would not knowledge that they had asleep. This is a light sleep phase and person can be awakened easily; this period usually lasts for several minutes and gives way to stage-II.

II. Asleep: In the Stage-II Sleep, theta waves become larger. Stage-II is characterized by two distinguishing phenomena: Sleep spindles and K-complexes. During stage-II spindle bursts appear in periodically. The appearance of spindles is usually a good sign that the person is in light sleep. K-complexes occur roughly every 1-7 minutes and are often followed by bursts of sleep spindles.

The K-complexes was discovered in 1937, by Alfred Lee Loomis. A K-complex is an EEG waveform that occurs during stage-II of NREM sleep. It is the largest event in healthy human EEG. K-complexes have two proposed functions: first, suppressing cortical arousal in response to stimuli that are sleeping brain evaluates, and second, aiding sleep based memory consolidation.

Sleepers pass through stage-II sleep several times during the night; more time is spent in stage-II sleep than in any other single stage, and typically constitutes about 45%-50% of total sleep time for adult (even more in young adults). 

III. Deep Sleep: During the course of the first hour or so after going to sleep the sleep becomes deeper. In deep sleep spindle bursts disappear and the record is characterized by trains of large and very slow delta waves. Now, at this stage of sleep subject is quite unresponsive, muscles relaxed heart rate and respiration rate fall lower.

IV. Stage-IV Sleep: This is defined by a continuous train of high voltage slow delta waves. It is very difficult to awaken someone during III & IV stages, which together are called deep sleep. There is no eye movement and muscle activity. People awakened during deep sleep do not adjust immediately and often feel groggy and disoriented for several minutes after they wake up.

Some children experience bed-wetting, night terrors or sleepwalking during deep sleep. Information processing and memory consolidation also takes place. Stage I to IV all classified as slow waves sleep.

REM Sleep: Apart from these, another unique phase of sleep has been discovered. The first REM sleep period usually occurs about 70 to 90 minutes after we fall asleep. A complete sleep cycle takes 90 to 110 minutes on average. The First sleep cycle each night contains relatively shorter REM periods and long periods of deep sleep. As the night progresses, REM sleep periods increases in length while deep sleep decreases. By morning, people spend nearly all their sleep time in stage I, II and REM sleep.

The records of this stage show a small voltage, fast activity, similar in many ways to that of the aroused or drowsy person that is why the name paradoxical sleep is given to this stage. Breathing and pulse rate becomes fast and quite irregular, and our arms and leg muscles becomes temporarily paralyzed, the eyes now show rapid movement under the closed eyelids, so this stage is also referred to as REM sleep or rapid eye movement sleep.

During this stage if the person is awakened he is very likely to report dreaming activities. In fact, dreaming is highly correlated with the REM stage. At the same time, the person is more difficult to arouse in this stage than he is in the slow wave phase of deeper sleep. The general conclusion is that the REM stage differs from other stages in that the cerebral cortex is more active than in deep sleep but at the same time thresholds for sensory stimuli are as high as they are in deep sleep.