Oxygen is vital to many organs and the performance of physiological functions. The brain, in particular, requires a certain amount of oxygen to work properly – approximately 20 percent of the body's oxygen supply – and this demand is met by the blood. However, when the brain is deprived of oxygen, it cannot metabolize glucose and convert it into energy. This leads to localized injuries or lasting brain damage, depending on the duration of hypoxia (oxygen deprivation). The effects of hypoxia on the brain have been extensively studied, but the effects of hyperoxia (excessive oxygen supply) have not. So the researchers decided to anesthetize naturally sleeping rats and administer high levels of oxygen to them. They used electroencephalography to record their brain wave patterns and interpret the effects.
Elevated levels of oxygen encourage deep sleep
When the researchers compared the effects of different levels of oxygen, they noticed that increasing the amount of oxygen by 100 percent prolonged the time the rats spent in SWS. They also noted that their heart rate and respiration rate decreased while in that state. On the other hand, when they introduced carbon dioxide and increased its concentration to about five percent, it decreased their time in SWS. Reducing the oxygen concentration also caused a steady decrease in the time they spent in SWS. When researchers lowered it to less-than-normal levels, they observed that the brain remained in active sleep. Doing this right after hyperoxia, interestingly, resulted in a rebound effect which caused the brain to go back to SWS for a longer duration afterwards. The researchers considered this an auspicious discovery because it further proved that elevated oxygen levels can encourage the brain to remain in or revert to deep sleep. They also believed that their findings highlight the clinical potential of oxygen therapy in helping people, especially those with sleep disorders, get enough restorative sleep. (Related: Better sleep can help to drastically reduce chronic pain.)
What is slow-wave sleep?
Slow-wave sleep is also known as stage 3 and stage 4 non-rapid eye movement (NREM) sleep. It is characterized in electroencephalograms (EEG) by slow, high-voltage waves called delta waves. SWS is part of NREM sleep or dreamless sleep where eye movement, breathing, and heartbeat are all relatively quiet and stable. In contrast, REM sleep or active sleep is where dreaming happens, usually accompanied by rapid eye movement, irregular breathing, vocalizations, and body movements. Humans who are in SWS are said to be in deep, restorative sleep. According to the lead author of the study, Brandon Hauer, "This seems to be the stage where metabolites are cleared from the brain, muscles grow, and proteins reform." SWS is known to promote brain restoration and recovery in humans. It also plays a role in memory consolidation during sleep. Staying in SWS for a longer period, therefore, may be beneficial to our brain's health.
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