Other than assuming a temporary “dead” state, one in which you’re predominantly immobile, unresponsive, unaware and disconnected from your usual conscious state, much much more happens beyond the visible, even from the perspective of an onlooker observing someone who’s asleep. Physical activity may be limited and inhibited but the physiological activity in your body certainly does not stop when you do fall asleep and graduate from one stage of sleep to the next… There’s more than meets the eye and here’s some deeper insights into what goes on behind the scenes when you’re sleeping…
What are the physiological bodily changes that occur during sleep?
Science has taken to great lengths to investigate the depths and heights of the great mystery called sleep and has come to prove various facts about your sleep, which includes both behavioral and physiological changes that differ from those present in your wakeful state.
Both the ANS (Autonomic Nervous System) and the CNS (Central Nervous System) undergo a series of changes while you sleep. The same goes for your cardiovascular system, your endocrine, sexual, renal, respiratory, gastrointestinal and metabolic systems.
ANS Changes During Sleep
In the first stage of your sleep, NREM sleep, key changes that affect your Autonomic Nervous System are a decrease in the sympathetic nervous activity, and an increase in the parasympathetic nervous tone.
In the consequent stage of your sleep, REM sleep, the main changes entail even more increase in the parasympathetic nervous tone, with an accompanying decrease in the sympathetic nervous activity. There is however a variation in the REM changes in that the activity of the sympathetic nerve is seen to be irregular whereby it increases sporadically.
Respiratory System Changes During Sleep
The respiratory system is highly vulnerable during episodes of normal sleep (hence the higher risk of upper airway obstruction and occlusion) as is elicited by the physiological changes that occur therein…
Neurons in the respiratory system are shown to exhibit a diminished firing rate for both stages of sleep, NREM and REM sleep. Even in the muscular tones present in the upper respiratory airways exhibit changes, whereby in NREM sleep there is a slight decrease but a significant decrease and consequently vanish in the REM sleep. This ultimately leads to higher resistance within the upper respiratory airways.
Ventilation responses are mildly diminished in your NREM sleep but more significantly reduced in your REM sleep. Ventilation in the alveoli as well as tension of the oxygen in the arteries is also diminished while tension of the carbon dioxide in the arteries is mildly increased in both your NREM sleep and REM sleep.
These respiratory changes further explain why asthmatic attacks are more prevalent, and their severity exacerbated while one sleeps due to the physiological changes that affect the constriction and ventilation of the bronchial airways.
Blood Circulation System Changes During Sleep
There is a marked decrease in your blood pressure, heart rate, cardiac output, and even resistance at the vascular periphery throughout the NREM sleep and this progresses onto the REM sleep where a further decrease is seen.
Blood flow to your brain as well as the cerebral metabolism of both oxygen and glucose is seen to diminish during NREM sleep; but the inverse happens in REM sleep whereby the levels ascend above those exhibited while one is awake.
These profound blood circulatory changes (erratic heart rates and blood pressure, continually diminishing cardiac output and the consequent maximum desaturation of oxygen, and intermittent breathing) go to explain why there is a higher incidence of early morning mortality cases (due to strokes and heart attacks) – particularly in patients with cardiac diseases.
Endocrine System Changes During Sleep
When you fall asleep, several paramount changes occur in your endocrine hormone system. Secretion of the growth hormone goes up in a pulsatile manner within the first third of your sleep which is NREM.
The hormone Prolactin is also shown to go up within the first 30 minutes to 90 minutes from the onset of your sleep.
In men, levels of the hormone Testosterone are seen to go up markedly from about 8:00 PM when they are lowest, to 8:00 AM when they are highest.
For the Thyroid Stimulating Hormone (TSH), the inverse happens whereby levels are at a peak high in the evening but gradually go down overnight.
On the other hand, secretion of the stress hormone Cortisol is shown to be inhibited altogether by the onset of sleep (that’s why sleep is good for dealing with stress).
From around 3:00 AM to 5:00 AM, secretion of the hormone Melatonin by the pineal gland attains peak levels, and then the hormone level gradually goes down over daytime hours.
Metabolism Changes during Sleep
As you fall asleep, the temperature of your body starts to drop with the onset of your sleep state, and it continues to drop till you attain the third portion of your sleep cycle when your body temperature is lowest.
Your body’s thermoregulation processes are active during your NREM sleep phase but however become inactive during your REM sleep phase when they are non-existent.
Sexual Reproductive System Changes during Sleep
It has been shown that erection of the male sexual organ (penis) is shown to take place during the REM stage of sleep, and the same goes for the female sexual organ, clitoris, whose tumescence also takes place in REM sleep.
Gastrointestinal System Changes during Sleep
While secretion of stomach acids, is normal and active during the day, it occurs variably when you’re asleep during both NREM and REM sleep. In addition, during the day, secretion of saliva in your mouth, as well as swallowing reflexes typically operate normally during the day, but these processes exhibit decreased activity during your NREM and REM phases of sleep.
In conclusion, it is more than evident that many physiological changes affect your body while you’re asleep. The master biological clock, the Circadian Rhythm, governs the sleep-wake cycles, working behind the scenes while you sleep.