The human body needs sleep to function properly and stay healthy. Some may think of sleep as a passive state where the brain turns off to rest, but this isn’t true. The brain stays active during sleep and may even be more active than when awake. This brain activity is broken up into several phases that, together, make up the sleep cycle. One cycle takes between one and two hours to complete and consists of rapid-eye-movement and non-rapid-eye-movement phases.
By using electroencephalography to view brain waves, scientists and researchers studying sleep discovered that brain activity varies throughout the night. Delta wave activity seems particularly variable. Certain bodily activities also change alongside the delta wave oscillation. When the delta wave activity increases during NREM sleep, secretion of the hormones responsible for lactation, blood pressure, and growth increase as well. During periods of decreased delta wave activity, such as the REM phase, thyroid-stimulating hormone secretions and heart rate variability increase.
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