As far as covetable drinks go, coffee probably ranks high up the ladder. We drink it for that instant boost of energy and to feel more awake. But for some of us, the plot twists: instead of an energy lift, the caffeinated drink leaves us feeling tired and suspiciously ready for a nap.
@thrivingmiddlechild no cause why does it do nothing but make me take the fattest naps 😭 #coffeetiktok #relatable ♬ original sound – random.shii089
What causes this stark difference in effect? Why does one person feel instantly energised after a cup of brew, while another starts experiencing a phenomenon popularly termed as the ‘coffee nap’ all over social media. According to Dr. Kenny Pang of Asia Sleep Centre, the answer may be more layered than we think. “Since coffee has a tolerance effect, meaning the more one drinks coffee, the less effect it has, one can become tolerant to it,” he says. Dr. Valerie Tay, senior consultant at Aspire Clinic, points to the bigger truth behind the dopamine boost. “The key thing is that caffeine doesn’t actually provide energy. Adenosine continues to build up while caffeine is in your system.”
@bevvgirl Why some people can drink coffee before bed and sleep just fine… #coffee #sleeptok #sleeptips #caffineaddict #nightcap ♬ original sound – Delaney | sleep content
So we put on our investigative hats and started asking the experts for the full lowdown. Here’s everything you need to know about the real deal with coffee and sleep.
To start, most of us think coffee wakes us up. So why do some people actually feel sleepy after drinking it?
According to Dr Kenny Pang and Dr Valerie Tay, the answer may come down to tolerance, sugar, dehydration, poor sleep quality, caffeine metabolism and the way caffeine interacts with the brain’s sleep-pressure system. “For most people, the caffeine in coffee is a stimulant, and does cause wakefulness,” says Dr Pang. “But coffee has a tolerance effect; meaning the more one drinks coffee, the less effect it has—one can become tolerant to it.”
Dr Tay explains the mechanism further. “Caffeine blocks adenosine receptors. Adenosine is a chemical that builds up while you’re awake and gradually makes you feel tired. It’s essentially your brain‘s sleep pressure signal.” She adds: “The key thing is that caffeine doesn’t actually provide energy. Adenosine continues to build up while caffeine is in your system. As the body metabolises caffeine, adenosine rushes back into its receptors all at once, causing a sudden wave of fatigue. For some, this shift happens faster than expected. In sleep-deprived people or those with certain genetic variants, the system can respond in ways that increase fatigue rather than relieve it.”
In simple terms, what’s happening in the brain when caffeine doesn’t have the energising effect we expect?
“You can think of adenosine as your brain’s “time to wind down” chemical. It builds up steadily throughout the day, making you feel sleepier as levels rise,” says Dr Tay. “When caffeine blocks adenosine receptors, it reduces feelings of drowsiness. But because caffeine doesn’t clear adenosine, the underlying sleep debt continues to grow while you use caffeine. This explains why caffeine can hide sleep deprivation but can’t resolve it. Eventually, sleep will become necessary, no matter how much caffeine you have. When caffeine clears after four to six hours, the accumulated adenosine floods back, often producing a harder crash than before. Regular coffee drinkers also grow more receptors over time, dulling caffeine’s effect overall,” she further explains.
Could genetics play a role in how someone reacts to coffee? Why might one person feel alert while another feels tired?
Dr Tay points to two genes in particular. “ Two important genes determine the metabolism of caffeine. The first gene, called CYP1A2, separates individuals into ‘fast’ and ‘slow’ metabolisers of caffeine. The second gene, known as ADORA2A, regulates sensitivity of the brain to the impact of caffeine; some genotypes are characterised by higher levels of caffeine-induced anxiety and problems with sleep. ADORA2A T alleles predispose individuals to experiencing caffeine-induced anxiety, whereas C alleles predispose one to having caffeine-induced sleep disorders. It means that if you have slow caffeine metabolism, caffeine will stay in your body longer, and thus the drop will be more significant. On the other hand, highly sensitive adenosine receptors may make you feel nervous or exhausted after consuming caffeine. While genetics is not the decisive factor in our reactions to coffee, they at least partly explain differences in reactions of people who have the same lifestyle and environment.”
Is it possible that coffee briefly masks tiredness and then makes it feel worse once it wears off?
“Yes, this also depends whether one has been “over-using” or “abusing” too much coffee,” says Dr Pang. Further, Dr Tay likens the effect to borrowing from an already depleted reserve. “The reason why it feels worse than the initial fatigue is that it’s like borrowing energy from one’s account: fatigue is already present; caffeine has just postponed payment.” She adds, “When someone is already sleep-deprived, then experiencing a crash after the flat white becomes overwhelming, especially compared to feeling tired initially.”
Some people say they feel calm or even relaxed after coffee. Is that normal, and what could explain it?
While caffeine is a stimulant, the experts point out that the wider ritual of consuming coffee may affect how people experience it. “Caffeine is a stimulant, it normally increases the heart rate, anxiety and alertness, but yes, some research shows that coffee can cause a calming effect, perceived to relieve stress,” says Dr Pang. “Some believe it is due to the fact that most would “sit down” to have coffee, hence, slowing the pace of the day.” Dr Tay agrees that the experience is not purely pharmacological. “Non-pharmacologic factors also matter: the aroma, warmth, and social context of coffee drinking can reduce perceived stress and produce comfort independent of caffeine.”
Could stress, anxiety, or burnout change how caffeine affects the body?
“Stress, anxiety, and burnout can change how some people respond to caffeine. Research suggests that caffeine may increase stress reactivity and worsen anxiety symptoms among sensitive individuals, and regular consumption of caffeine has been linked to greater cortisol response under stress in some studies.If you already feel exhausted, caffeine, if consumed too late or in higher amounts, may disrupt sleep and leave you feeling more fatigued the next day,” elucidates Dr Tay.
Does drinking coffee on very little sleep make this reaction more likely?
Dr Tay explains, “The process of upregulation of adenosine receptors in the brain speeds up due to lack of sleep as a compensatory response to prolonged wakefulness. Caffeine blocks adenosine receptors and can reduce sleepiness, but it does not prevent adenosine accumulation, so it cannot fully prevent the effects of severe sleep deprivation.” She adds, “Therefore, the dose of caffeine required increases for the same effect, the rebound effect increases, and its consumption in the evening interferes with sleep during the next day.”
Can drinking coffee regularly build up a tolerance that changes how it affects our energy levels?
“The regular amount of one to two cups of coffee per day should be alright. If one consumes large amounts of coffee daily, they might build up tolerance to it,” says Dr Pang. Dr Tay explains what may be happening in the body. “With regular caffeine consumption, the body adapts to the constant blockage of adenosine receptors upregulation of these receptors. In most people, this is the primary cause of caffeine tolerance, where you may need more caffeine to achieve the same effect.” She notes, “A 10 to 14 day break can reset this in most individuals.”
Does timing matter? For example, could having coffee during an afternoon slump make someone feel more aware of their tiredness?
There is some difference in belief here. Dr Pang is less convinced that an afternoon coffee would make someone more aware of fatigue. Dr Tay, however, notes that timing can still matter. “There is a natural circadian dip in alertness between roughly 1pm and 3pm that caffeine cannot effectively counter, since adenosine levels are already high. Additionally, caffeine has a half-life of five to seven hours, meaning a 3pm coffee still has half its concentration at 10pm, directly suppressing REM sleep and increasing fatigue the following day.”
When someone tells you coffee makes them sleepy, what does that signal to you about their sleep health overall?
“No. It just could be due to tolerance or poor sleep habits,” says Dr Pang. “Sleep hygiene is very important.” While Dr Tay sees it as something worth paying attention to. “It’s a meaningful clinical flag. Caffeine doesn’t truly “create energy”, as we have mentioned above, it blocks adenosine signalling. Chronic sleep debt is the most common explanation, with adenosine levels so persistently high that caffeine can’t fully overcome them.”
