Sleep is a natural condition that occurs regularly and in which one loses awareness of one's surroundings. Yet it is still largely a mystery as to what sleep actually is. It is known, however, that there are basically two kinds of sleep.
In one kind, which starts with drowsiness and gets deeper and deeper, profound restorative processes take place. In it your breathing and your heartbeat slow down, your blood pressure drops and your limbs fully relax. This deep sleep is believed to be an aid to memory. It lasts for about ninety minutes.
Afterwards, you return to a sleep much lighter in some respects, and in other respects deeper. It is called the REM stage because it is marked by side-to-side Rapid Eye Movements, which indicate that you are dreaming. Your heartbeat greatly fluctuates and your limbs grow tense, indicating that not only your mind but also your body is involved in dreaming. After about ten minutes of this REM sleep you will again go into deep sleep for another ninety minutes and then back up, and so on throughout the night.
Most sleep researchers hold that both kinds of sleep are essential for mental and physical health; one cannot take the place of the other
In one kind, which starts with drowsiness and gets deeper and deeper, profound restorative processes take place. In it your breathing and your heartbeat slow down, your blood pressure drops and your limbs fully relax. This deep sleep is believed to be an aid to memory. It lasts for about ninety minutes.
Afterwards, you return to a sleep much lighter in some respects, and in other respects deeper. It is called the REM stage because it is marked by side-to-side Rapid Eye Movements, which indicate that you are dreaming. Your heartbeat greatly fluctuates and your limbs grow tense, indicating that not only your mind but also your body is involved in dreaming. After about ten minutes of this REM sleep you will again go into deep sleep for another ninety minutes and then back up, and so on throughout the night.
Most sleep researchers hold that both kinds of sleep are essential for mental and physical health; one cannot take the place of the other