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Even if you sleep for many hours, you may still wake up feeling heavy and mentally foggy. In such cases, the total amount of sleep is not the most important factor. What matters is whether you are getting enough deep sleep—also known as slow-wave sleep (a stage of non-REM sleep characterized by slow, high-amplitude brain waves). What happens when this crucial phase, during which both the body and brain recover, is insufficient?

◇ Impaired Blood Sugar Control

A lack of slow-wave sleep can make blood sugar levels fluctuate more easily the next day. According to a study published in Psychoneuroendocrinology by researchers at the University of Lübeck in Germany, when slow-wave sleep in 16 healthy men was disrupted using sound stimuli, their blood glucose and insulin levels rose more sharply after drinking a glucose solution the following day. In addition, post-meal insulin sensitivity decreased by up to 20%. In contrast, disrupting REM sleep (the stage associated with dreaming) did not produce such significant changes. This suggests that even if you sleep long enough, insufficient deep sleep can negatively affect blood sugar control.

◇ Increased Blood Pressure

Insufficient deep sleep may also prevent blood pressure from dropping adequately overnight. A study published in Sleep followed 1,850 adults without hypertension for an average of 5.3 years. Those whose slow-wave sleep accounted for less than 9.8% of total sleep had a 69% higher risk of developing hypertension compared to those with 17.7%–25.2%. This highlights that sleep quality, like dietary habits such as salt intake, is an important factor in blood pressure management.

◇ Disrupted Long-Term Memory Formation

During sleep, the brain organizes and consolidates information learned throughout the day. Slow-wave sleep plays a critical role in transferring new information into long-term memory. A study published in Science by researchers at the University of Lübeck found that when participants were exposed to a specific odor during slow-wave sleep—after learning a spatial memory task—they showed improved recall of spatial locations the next day. This enhancement in declarative memory (memory that can be consciously recalled and described) did not occur when the same cue was presented during wakefulness or REM sleep.

◇ Increased Risk of Dementia

A reduction in slow-wave sleep has also been associated with a higher risk of dementia. According to a study in JAMA Neurology, 346 adults aged 60 and older underwent sleep studies twice at approximately five-year intervals and were followed for up to 17 years. The findings showed that for every 1 percentage point annual decline in slow-wave sleep, the risk of developing dementia increased by 27%.

◇ How to Increase Slow-Wave Sleep

To increase slow-wave sleep—and improve overall sleep quality—regular daytime exercise is highly beneficial. A study published in the Journal of Applied Physiology by researchers from Waseda University and others found that on days when 14 healthy men who were typically inactive performed moderate-intensity aerobic exercise between the morning and early evening, their slow-wave sleep increased by 33% compared to non-exercise days. Rather than intense workouts, consistent activities such as brisk walking or indoor cycling that slightly elevate your breathing are recommended.

It is also best to avoid alcohol and late-night meals before going to bed. While alcohol may help you fall asleep faster, it can increase nighttime awakenings and disrupt deep sleep. Reducing caffeine intake in the evening is equally important. Caffeine is found not only in coffee but also in green tea, black tea, and chocolate. Keep your bedroom cool, and if you experience severe snoring or episodes of breathing interruption during sleep, consider being evaluated for sleep apnea.

Source: 헬스조선 (https://health.chosun.com/site/data/html_dir/2026/07/28/2026072802189.html)