Acute caffeine intake improves muscular strength, power, and endurance performance, reversing the time-of-day effect regardless of muscle activation level in resistance-trained males: a randomized controlled trial

Caffeine improves physical performance more in the morning than in the afternoon because core body temperature starts lower in the morning and rises more after caffeine intake.

Caffeine makes people feel like they are working harder during exercise and increases their motivation to work out, but it does not cause muscles to grow unless the physical load on the muscles is increased.

Consuming 3 mg of caffeine per kilogram of body weight before exercise increases the speed and power output during back squat endurance tests at 65% of one-rep max by 6–9% in trained men, regardless of time of day, and increases performance by the same amount in the morning during bench press tests.

Taking 3 mg of caffeine per kilogram of body weight does not change the electrical activity in the pectoralis major, triceps brachii, rectus femoris, or vastus lateralis muscles during strength, power, or endurance exercises in trained men.

When resistance-trained males consume 3 mg/kg of caffeine before performing back squats at 75% or 90% of their one-repetition maximum, their average movement speed and power output increase by 2.5 to 3.8 standard deviations, regardless of the time of day.

Drinking caffeine in the morning increases core body temperature and enhances physical performance. Regular caffeine use over time does not affect muscle growth because the body adapts to its presence.