Caffeine's ergogenic effect is greater in the morning than in the afternoon due to a larger increase in core body temperature from a lower baseline in the morning compared to a higher baseline in the afternoon.

Study: Differential Time-of-Day Effects of Caffeine Capsule and Mouth Rinse on Physical Performance and Volleyball-Specific Skills in Adolescent Male Volleyball Players

Caffeine helped athletes perform better in the morning and afternoon than at night, which matches the idea that it works better when your body is naturally cooler — like in the morning.

Study: Effects of Different Caffeine Dosages on Maximal Physical Performance and Potential Side Effects in Low-Consumer Female Athletes: Morning vs. Evening Administration

Caffeine helped athletes perform better in the morning but not in the evening, which matches the idea that it works better when your body is cooler in the morning.

Study: Improvements on neuromuscular performance with caffeine ingestion depend on the time-of-day.

Caffeine helps people perform better in the morning than in the afternoon because your body is naturally cooler in the morning, and caffeine warms you up just enough to boost your muscles. In the afternoon, when you're already warmer, caffeine doesn't help as much—and can even make you feel worse.

Study: Caffeine ingestion attenuates diurnal variation of lower-body ballistic performance in resistance-trained women

Caffeine helped women jump higher in the morning more than in the afternoon, suggesting it works better when your body is naturally cooler in the morning.