Coffee and Dennis, by Doug Bloodworth (oil on canvas)
Even when it doesn’t cause insomnia, caffeine impairs sleep quality
STUDY: Chmiel J et al, Nutrients 2026
STUDY TYPE: Systematic review
FUNDING: Wrocław Medical University (Poland)
Background
Inside the body are two natural sleep “medicines.” Melatonin regulates circadian rhythm, rising in the dark and shutting down in light. Adenosine regulates sleep drive, rising the longer we stay awake.
Caffeine keeps us awake by blocking adenosine. It’s potential to cause insomnia is well known, and this review looks at how it changes sleep quality, especially the slow waves of deep sleep.
The Study
- 32 human studies, published from 1980 to 2026, that recorded sleep EEG after caffeine exposure.
- Designs included bedtime and evening doses, repeated daytime use, recovery sleep after sleep deprivation, withdrawal, and home EEG monitoring.
- Most doses ranged from 100 to 400 mg (about one to four cups of coffee).
- Samples were small (8 to 47 people) and mostly healthy young men.
Results
The most consistent finding was that caffeine reduced slow-wave activity, the deepest delta waves of non-REM sleep. It raised faster waves linked to arousal, leaving a lighter, more wake-like sleep, particularly in the first few hours of sleep. The effects were more prounounced after sleep deprivation, when sleep drive (adenosine) is highest.
Deep sleep was reduced with caffeine by:
- 42% in the first 3 hours after 300 mg at bedtime (adults aged 50 to 63)
- 32% after 400 mg taken 6 hours before bed (home recordings)
- 9% after 80 mg in the evening (adolescents)
Even 100 mg (1 cup coffee) at bedtime reduced early-night deep sleep, and 200 mg in the morning still altered nighttime EEG. In one home study, 100 mg taken 4 to 12 hours before bed had no significant effect, while 400 mg disrupted sleep at every interval.
Higher doses, later timing, prior sleep loss, stress-reactive sleep, and variants in the adenosine receptor gene (ADORA2A) all made people more vulnerable. Tolerance to these effects was incomplete. Habitual users often reported sleeping well while their EEG still showed lighter sleep. REM effects were less consistent.
Practice Implications
- Caffeine may impair sleep even when we’re not aware of it, causing more fatigue the next day.
- A single morning coffee, however, looks safe for most people, while tea is safer, with more brain benefits and ingredients that improve sleep.
- Taper by one cup every few days to avoid withdrawal headaches and fatigue.
—Chris Aiken, MD
Director, Psych Partners
Editor in Chief, Carlat Psychiatry Report
