Late night, asleep at 11 pm, wake up at 7 am, a full 8 hours. But as the alarm goes off, still feeling too tired to move. Eyes open, but the brain seems to still be lingering in the bed, washing the face doesn’t bring any refreshment, only after a cup of coffee does the day officially begin.
Even though having a full 8 hours of sleep, why still feel so tired? The issue might not just be about how long one sleeps.
A new study involving 95,559 people has found that two individuals both sleeping for 7-8 hours, one sleeping through the night and the other waking up several times, tossing and turning, these two nights couldn’t be judged solely based on time. Aside from sleep duration, factors such as deep sleep, rapid eye movement sleep (REM), how many times one wakes up during the night, and the regularity of sleep all have links to future health conditions.
The study itself didn’t directly measure how tired individuals felt the next day, but it revealed that focusing only on sleep duration would miss out on many other important aspects.
This study was published on September 17 in the medical journal “PLOS Medicine”. Researchers used data from 95,559 participants from the UK Biobank who wore wrist accelerometers for 7 consecutive days, even during sleep. Using a deep learning algorithm SleepNet, the researchers estimated how long they slept, how long they were awake at night, the proportions of different sleep stages, and the regularity of sleep, then tracked their health conditions for a median of 8.9 years.
Looking at what many people care about most, sleep duration, among 86 diseases that exhibited a non-linear relationship with sleep duration, the lowest risk for 69 of those diseases fell within the range of 6 to 8 hours of sleep. Sleeping too little poses more problems: the study found that sleeping fewer than 5 hours per night was associated with an increased risk of 37 diseases.
In other words, sleeping for 8 hours is not a strict pass-or-fail line. At least from this study’s perspective, for middle-aged and elderly individuals, sleeping for 6 to 8 hours per night is considered a more ideal range.
But this still doesn’t explain another issue: why are some individuals still tired the next day despite sleeping for 7-8 hours?
Breaking down sleep patterns reveals the differences.
When people fall asleep, they don’t remain in the same state throughout the entire night. Throughout the night, they cycle between light sleep, deep sleep, and rapid eye movement sleep.
Rapid eye movement sleep is commonly referred to as REM sleep. During this time, the brain is more active, individuals are more prone to dreaming, and it is related to memory, learning, and emotional regulation. Deep sleep, on the other hand, is the most restful stage of sleep, crucial for bodily rest and recovery.
The study found that individuals with more REM sleep had a lower risk of 83 diseases, including heart failure, dementia, and Parkinson’s disease; having more deep sleep was also associated with a lower risk of 7 diseases, including type 2 diabetes and severe depression.
On the flip side, having longer periods of wakefulness after falling asleep was linked to an increased risk of 6 diseases; inconsistent sleep duration from day to day was associated with an increased risk of 3 diseases.
This indicates that even though two individuals both “slept for 8 hours,” there could be significant differences in their sleep quality.
One person goes to bed at 11 pm, wakes up at 7 am with minimal awakenings in between; another also sleeps from 11 pm to 7 am but wakes up several times during the night, feeling hot, needing to use the bathroom, tossing and turning, unable to fall back asleep. Both individuals may tally up 8 hours the next day, but their sleep quality during that night is not the same.
Therefore, if feeling tired even after a full 8 hours of sleep, the issue might not necessarily be “not sleeping enough,” but perhaps having a fragmented night’s sleep.
However, if one is wearing a smartwatch, they should not obsess over the numbers for “deep sleep” and “REM” every day. This study used wrist devices and algorithms to estimate sleep stages, which is not the same as determining sleep stages in a sleep lab using methods like electroencephalography. The numbers on a watch can be used as a reference but not for diagnosing sleep issues yourself.
Since insufficient sleep is not good, does that mean sleeping for 9 or 10 hours every day is healthier?
This study doesn’t support such a simple algorithm. The relationship between sleep duration and the risk of many diseases is not a case of “the longer you sleep, the lower the risk.” The lower-risk range for many diseases often falls within the 6 to 8-hour window.
Therefore, if someone sleeps for 6.5 or 7 hours each night and feels energetic during the day, they may not necessarily need to force themselves to hit that “8-hour” mark early every night. Rather than fixating on a single number, it’s more crucial to pay attention to whether one wakes up frequently at night, if their sleep duration varies greatly day to day, and how well-rested they feel during the day.
There is one aspect in the study that might be misinterpreted: having more REM sleep was associated with a lower risk of 83 diseases. This does not mean that “increasing REM sleep can prevent 83 diseases.”
This is an observational study, noting the link between sleep characteristics and diseases, it doesn’t prove that any specific type of sleep directly causes or prevents a particular disease. Some diseases may affect a person’s sleep patterns early on, so the causal direction cannot be determined conclusively.
Moreover, the study participants were mainly middle-aged and elderly individuals, so these findings cannot be directly applied to all age groups; the wrist devices were worn continuously for only 7 days, recording the sleep patterns during that period, and may not represent how a person has been sleeping over many years.
It’s not uncommon to feel groggy the next day after a single night of poor sleep. However, if someone consistently carves out 7-8 hours for sleep each day but still feels excessively tired during the day, wakes up frequently at night, snores loudly, or even struggles to breathe while sleeping, just going to bed earlier each night may not be the solution.
Sleep apnea is a classic example of this. Some people may not even realize they have it; although the watch might show “slept 8 hours last night,” their breathing repeatedly gets obstructed, their sleep is interrupted, and they might still feel very tired the next day.
So, the next time your watch displays “slept 8 hours last night,” don’t rush to assume that you had a restful night’s sleep.