- A recent review has found that even low doses of caffeine may affect sleep quality, particularly slow-wave (deep) sleep.
- While caffeine consumed closer to bedtime has a stronger impact, consuming caffeine in the morning may still reduce sleep, the researchers found.
- Experts advise that genetics, age, and metabolism may also influence how caffeine affects sleep, so there’s no universal cutoff time.
Caffeine is generally associated with increased alertness while awake, but its effects may persist while sleeping.
Caffeine affects adenosine signaling, which is vital in regulating the sleep-wake cycle. Now, researchers are investigating whether caffeine consumption impacts the brain during sleep.
A systematic review published in Nutrients assessed the effect of caffeine on sleep-related electroencephalography (EEG), a test that records the electrical activity in the brain. In particular, they looked at how caffeine might affect low-frequency brain waves associated with deep sleep.
Deep sleep occurs during the
The researchers found that caffeine consumption could negatively impact the quality of deep sleep, potentially reducing the body’s ability to repair itself during this time.
Research findings suggest that caffeine suppresses deep, restorative sleep, while increasing faster brain wave activity.
The review focused on 32 human studies from January 1980 to January 2026. The researchers used EEG data to assess the effects of caffeine on the brain’s electrical activity during sleep.
The most consistent finding was a reduction in low-frequency NREM sleep EEG activity. For example, in
Caffeine also appeared to increase faster EEG activity, suggesting that it may lead to lighter, more wakeful sleep.
Medical News Today spoke with three experts to learn more about the possible link between caffeine — including the amount and the time of consumption — and its effects on deep sleep.
The researchers found that caffeine doses as low as 100 milligrams (mg) were sufficient to affect slow-wave activity early in the night.
“Generally, the more caffeine you consume, the greater the potential impact on sleep,” said Steven Allder, MD, consultant neurologist at Re:Cognition Health, in an interview with Medical News Today.
“The latest review found that higher doses can reduce slow-wave brain activity, which is closely linked to deep sleep. That does not mean one extra coffee will automatically ruin your sleep, but larger amounts are more likely to interfere with the brain’s normal sleep patterns,” Allder, who was not involved in the research, explained.
Şebnem Ünlüişler, MSc, Genetic Engineer at London Regenerative Institute, who was not involved in the research, agreed that, “[i]n general, higher doses are more likely to affect sleep, although timing and individual sensitivity also matter.”
The review suggests that caffeine consumed closer to bedtime has a greater effect on deep sleep than caffeine consumed earlier in the day.
However, morning caffeine can still impact sleep.
Another study found that consuming 400 mg around 6 hours before bed reduced slow-wave sleep from 71.5 minutes to 48.9 minutes.
However, the same study also found that taking the same amount of caffeine right at bedtime reduced slow-wave sleep to 56.7 minutes, a smaller reduction, though caffeine still affected deep sleep.
Ünlüişler told MNT that “[e]vening caffeine is more likely to interfere with sleep because the body has less time to clear it.”
Allder added, “Even caffeine consumed several hours before bed can have an effect, particularly at higher doses. Morning coffee is generally less likely to cause problems because the body has much longer to metabolize it before bedtime.”
Caffeine does not affect sleep in the same way for everybody.
It is important to note that the review authors highlight limitations in the studies they included. In particular, older adults, adolescents, and people with sleep problems or psychiatric vulnerability were underrepresented.
However, they were able to determine that variations in the ADORA2A gene could affect how caffeine affects sleep quality. The authors note that a one-size-fits-all recommendation for the timing and amount of caffeine consumption would not account for people who may be more sensitive to caffeine.
Ünlüişler explained that “[g]enetics can play an important role in how we respond to caffeine. CYP1A2 influences how quickly caffeine is metabolized, while ADORA2A can affect how sensitive the brain is to caffeine’s effects on adenosine signaling and sleep.”
“Age, medications, habitual caffeine intake, and lifestyle also matter, which is why the same amount of caffeine can affect two people very differently.”
— Şebnem Ünlüişler, MSc
Allder added that everybody responds to caffeine differently. “Your sensitivity, age, habitual caffeine intake, how quickly you metabolize caffeine, and when you drink it can all make a difference. Some people can have a coffee in the afternoon and sleep soundly, while others may notice a difference from even a lunchtime coffee.”
There is “no universal cut off time” for enjoying caffeine without affecting sleep quality, said Ünlüişler.
“A good approach is to keep most caffeine to the morning or early afternoon and move your last caffeinated drink earlier if you notice it affecting your sleep,” she said.
Allder recommended thinking “about how much you are having and when you are having it. Keeping caffeine mainly to the morning and early afternoon is a simple way to reduce the chance of it interfering with sleep.”
“If you are waking tired or your sleep feels less restorative, try moving your last caffeinated drink earlier and see if it makes a difference. You do not necessarily need to give up caffeine, but it is worth finding the cut-off time that works for your own body.”
— Steven Allder, MD
