Coffee helps millions of people feel more alert, but its effects may continue long after the initial energy boost disappears. New research suggests that even relatively small amounts of caffeine can alter brain activity during deep sleep.

A systematic review published in the journal Nutrients examined how caffeine consumption influences sleep patterns. The findings indicate that doses as low as 100 milligrams may affect the brain waves associated with restorative sleep.

However, individual responses vary considerably, and drinking coffee does not automatically mean someone will experience poor sleep.

What Scientists Discovered

Researchers analysed 32 human studies conducted between January 1980 and January 2026.

They focused on electroencephalography (EEG), a technique that records electrical activity in the brain. The most consistent finding was a reduction in slow-wave activity during non-rapid eye movement sleep. Slow-wave activity is particularly important during deep sleep, when the body carries out essential restorative processes.

In one study involving middle-aged and older adults, caffeine consumed before bedtime reduced time spent in a particular stage of non-REM sleep from 66 to 38 minutes during the first three hours of the night.

Researchers also observed increased faster-frequency brain activity, suggesting that caffeine can make sleep less restful.

Even Small Amounts May Matter

The review found evidence that 100 milligrams of caffeine could influence slow-wave brain activity early in the night.

That amount is comparable to the caffeine contained in some ordinary cups of coffee, although concentrations vary considerably. Larger doses generally produced more noticeable effects. However, the evidence does not establish that every 100-milligram serving meaningfully disrupts sleep.

Another controlled trial found no significant overall sleep disturbance from a 100-milligram dose taken four hours before bedtime, highlighting the importance of the outcome being measured.

Can Morning Coffee Affect Sleep?

Many people assume caffeine consumed early in the day cannot interfere with sleep that night. Yet one small study found that taking 200 milligrams shortly after waking was associated with shorter total sleep time and reduced sleep efficiency.

Participants consumed caffeine at approximately 7.10am before going to bed at 11pm. Another experiment found that 400 milligrams taken six hours before bedtime reduced slow-wave sleep from 71.5 to 48.9 minutes.

These findings suggest that the effects of caffeine can persist much longer than its noticeable stimulating sensation.

Why People Respond Differently

Caffeine affects the activity of adenosine, a chemical involved in regulating sleepiness. However, the speed at which the body processes caffeine differs between individuals.

Genetic variations may help explain these differences. The CYP1A2 gene influences caffeine metabolism, while ADORA2A is associated with sensitivity to its effects on sleep. Age, medication use and habitual caffeine consumption may also influence individual responses.

The researchers acknowledged limitations in the available evidence, including insufficient representation of adolescents, older adults and people with existing sleep difficulties.

How To Protect Your Sleep

Experts recommend paying attention to both caffeine quantity and consumption time. Keeping caffeinated drinks to the morning or early afternoon may reduce the likelihood of sleep disruption. However, there is no universal cut-off time suitable for everyone.

People who regularly wake feeling tired could experiment with drinking their last coffee earlier and monitoring whether their sleep improves.

Finding A Healthy Balance

The findings do not suggest that everyone needs to stop drinking coffee. Instead, they highlight how caffeine can influence sleep in ways that may not be immediately noticeable.

Understanding personal sensitivity and adjusting daily habits could help people enjoy their favourite drinks while protecting the restorative sleep their bodies need.