According to the Centers for Disease Control and Prevention (CDC), adults should aim for at least seven hours of sleep each night.
But specialists emphasize that sleep quality depends on how the brain moves through its repeating cycle of light NREM sleep, deep NREM sleep, and rapid eye movement (REM) sleep.
According to the National Institutes of Health a full cycle lasts roughly 90 to 110 minutes.
Light sleep typically opens each cycle and helps the body settle. REM sleep, which appears later, is tied to dreaming and emotional processing.
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The deepest NREM stage is considered the most physically restorative, supporting tissue repair, immune function, and hormonal balance.
How much deep and REM sleep adults tend to get
Sleep researchers from the National Institutes of Health generally find that healthy adults spend about a quarter of the night in deep NREM sleep.
For someone who sleeps seven to eight hours, that usually amounts to around an hour and a half to two hours.
Another 20–25 % is commonly spent in REM sleep, though individual patterns vary.
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As people age, they often transition toward lighter stages and naturally accumulate less deep sleep.
Light sleep has no recommended minimum; it appears in every cycle and is nearly impossible to avoid.
Why restorative sleep can be hard to reach
Even with enough total hours, some people wake feeling unrefreshed.
Sleep medicine specialists note that irregular schedules, stress, nighttime discomfort, travel across time zones, or conditions such as sleep apnea can interrupt deeper stages.
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Experts recommend steady routines, limited caffeine late in the day, and a dark, quiet room to improve sleep continuity.
If persistent fatigue continues, clinicians often suggest a sleep study to check for medical causes.
Sources: Healthline, Centers for Disease Control and Prevention, and National Institutes of Health.
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