Why Coffee Makes Some People Sleepy Instead of Energised, According to Science

For many people, coffee provides an instant boost of alertness. However, some individuals experience the opposite effect, feeling calm, relaxed, or even sleepy after consuming caffeine. Scientists say this reaction is not unusual and can be explained by how caffeine interacts with the brain and body.

One of the biggest misconceptions about coffee is that it directly gives energy. In reality, caffeine works by blocking adenosine, a brain chemical that builds up throughout the day and signals fatigue. By preventing adenosine from attaching to its receptors, caffeine temporarily masks feelings of tiredness, creating the sensation of increased energy and alertness.

However, the body continues producing adenosine even while caffeine is active. Once the caffeine’s effects wear off, the accumulated adenosine can bind to its receptors all at once, leading to what experts call the “adenosine rebound” effect. This can cause sudden fatigue and sluggishness, particularly in people who are already sleep-deprived.

Genetics also play a significant role in determining how coffee affects an individual. Variations in the CYP1A2 gene influence how quickly caffeine is metabolised in the liver. Fast metabolisers may experience caffeine’s effects for only a short period before feeling tired again, while slow metabolisers may remain alert for longer. Another gene, ADORA2A, affects caffeine sensitivity by influencing adenosine receptors in the brain.

Coffee can also affect dopamine, a neurotransmitter linked to attention, motivation, and reward. In some people, especially those who are easily distracted or mentally overstimulated, caffeine can improve focus and create a sense of calm rather than excitement. This may explain why some individuals feel relaxed after drinking coffee.

Researchers have also observed unique caffeine responses among people with Attention Deficit Hyperactivity Disorder (ADHD). Although caffeine is not a treatment for ADHD, it can affect dopamine pathways and may help some individuals improve concentration or feel calmer.

Regular coffee drinkers may also develop caffeine tolerance over time. As the brain adapts to repeated caffeine exposure, the stimulating effects become less noticeable, meaning a daily cup of coffee may simply restore normal alertness rather than provide an energy boost.

Experts say there is no universal response to caffeine. While some people feel energised, others may feel calmer, more focused, or even sleepy, depending on their biology, genetics, and caffeine habits. (Agencies)

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