In female handball players aged 18–19 with low habitual caffeine consumption, evening ingestion of 6 mg/kg of caffeine results in a higher frequency of side effects (insomnia, headaches, tachycardia) compared to morning ingestion of the same dose, while performance outcomes remain similar between times of day.

When caffeine is taken in the evening, it blocks signals that normally calm the nervous system and stop the brain from releasing stress hormones. This causes the heart to beat faster, blood vessels to tighten, and the brain to stay alert when it should be preparing for sleep. At the same time, caffeine stops the body from making the sleep hormone melatonin, making it harder to fall asleep. These effects together cause headaches, rapid heartbeat, and insomnia. In the morning, the same dose of caffeine boosts alertness without disrupting sleep because the body is naturally waking up, so the stress response does not trigger the same side effects.