How Coffee Affects Your Sleeping Brain – and Why Young Adults Are Worse Off
In a new study published in the journal Nature Communications Biology, a team of researchers explored how caffeine affects sleep and brain recovery – physically and cognitively – overnight.
The team studied a total of 40 healthy adults, whose brain activity was measured on two separate nights: one when they consumed caffeine capsules three hours and one hour before bedtime (totalling 200mg caffeine), and another when they took a placebo at the same times. The scientists then used AI and electroencephalography (EEG) to study the participants' nocturnal brain activity to see how caffeine affected their sleep.
Following their statistical analysis, the researchers found that caffeine increased the complexity of the participants' brain signals, making them more dynamic and less predictable – particularly during the non-rapid eye movement (NREM) phase of sleep, which is crucial for memory consolidation and cognitive recovery. They also identified changes in the brain's electrical signals during sleep. Namely, that caffeine led to slower oscillations (such as theta and alpha waves, which are generally associated with deep, restorative sleep) and stimulated beta wave activity – seen more during wakefulness and mental engagement.
'These changes suggest that even during sleep, the brain remains in a more activated, less restorative state under the influence of caffeine,' said Karim Jerbi, a psychology professor and one of the study's authors. 'This change in the brain's rhythmic activity may help explain why caffeine affects the efficiency with which the brain recovers during the night, with potential consequences for memory processing.'
The main thing you need to know is that, according to this study, caffeine increases brain signal complexity and enhances something called your brain 'criticality' during sleep. Jerbi defined this as 'a state of the brain that is balanced between order and chaos'.
'It's like an orchestra: too quiet and nothing happens, too chaotic and there's cacophony. Criticality is the happy medium where brain activity is both organised and flexible. In this state, the brain functions optimally: it can process information efficiently, adapt quickly, learn and make decisions with agility.'
When we drink coffee, the brain is stimulated and pushed into a state of criticality – where it's more awake, alert and reactive. 'While this is useful during the day for concentration, this state could interfere with rest at night: the brain would neither relax nor recover properly,' explained Julie Carrier, a sleep and ageing professor whose team collaborated with the researchers on this study.
To ensure your brain gets ample recovery time while you sleep, it could mean reconsidering that 4 pm work slump coffee and swapping it for something lower in caffeine or free from it altogether. Government guidelines suggest around 400 mg of caffeine as the upper daily limit. Given that the participants in this study consumed 200 mg in the three hours before bedtime and still experienced increased brain activity during sleep, you might benefit from rethinking your coffee schedule and potentially shifting it to front-load your caffeine in the first half of the day.
According to this study, your sleeping brain may be less affected by coffee the older you get. Researchers found that younger adults (aged 20-27) compared to middle-aged participants (41-58) were significantly more affected by caffeine – especially during REM sleep – the phase associated with dreaming. They said this increased response was likely due to a higher density of adenosine (a molecule that gradually accumulates in the brain throughout the day, causing feelings of fatigue) receptors in their brains.
'Adenosine receptors naturally decrease with age, reducing caffeine's ability to block them and improve brain complexity, which may partly explain the reduced effect of caffeine observed in middle-aged participants,' Carrier explained.
This was a relatively small study and the researchers highlighted that while it's the first to show that caffeine enhances brain criticality during sleep, more research is needed to confirm the results.
Now they've shed light on what happens in the brain after lights out following caffeine consumption, the researchers said more studies are needed to explore how these neural changes might affect people's cognitive health and daily functioning – and to potentially guide personalised recommendations for caffeine intake.
Thanks to its antioxidant properties, coffee can have several health benefits, including reduced risk of chronic diseases like heart disease and some types of cancer. So, this isn't to say it's 'bad' for your health.
Rather, this study's unique insight into neural activity gives us some things to consider regarding when, why and how we consume caffeine to ensure our brains get the most out of a good night's kip.
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