Our body runs on an internal clock, which controls most functions. We become tired and sleepy at a certain time every evening and we usually awaken at the same time every morning. Hunger strikes around the same time every day. These and many other functions follow a daily rhythm of peaks and low points known as a circadian (daily) rhythm. Our sleep/wake pattern as well as daylight/darkness set this rhythm pattern. Let's assume you have a traditional sleep schedule of bedtime 10-11pm and waking 6-7am. If you awaken earlier or later then 6:00 shift the times accordingly. Also note that there are other variables such as the "early bird" rhythm and the "night owl" rhythm. Some individuals have their peaks earlier in the day and function better early. They tend to be natural early risers. Others have their peaks later in the day. They tend to work better later in the evening. Below is a generalization of circadian rhythm. Cortisol appears the be a principle hormone which has a natural 24 hour rhythm which peaks early in the day (8-10am) and begins to taper in the afternoon (3-4pm) and is at its lowest in the dead of the night (4-6am).
1-6am: Body temperature and heart rate hit their daily low, making sleep almost irresistible for anyone still awake.
6-9am: The body burns calories the fastest: a large, high protein meal eaten now not only metabolizes quickly but also provides energy for the day. Sex hormones are at their highest levels too, promoting arousal.
9am-noon: Levels of stress hormones like cortisol peak. This increases analytical skill and mental performance (so try balancing the books) but decreases the ability to relax.
Noon to 2pm: Alertness and verbal reasoning peak. We also tolerate physical discomfort more during late mornings. Also provides energy for the day.
2-3pm: A post-lunch slump, caused not by a meal but by the rhythmic function of the brain, increases episodes of drowsiness, daydreams and lapses in concentration.
3-7pm: Alertness rises again, and long-term memory peaks. Stress hormone levels decline, allowing muscles to relax. Heart rate, blood pressure and body temperature rises. Now is the best time for sports.
7-9pm Body temperature begins its descent. The body's reflexes and thinking skills are winding down. Sensory perceptions peak about now, so appreciation of music or fine wine and food is enhanced.
9-11pm: As body temperature drops sharply, sleepiness is likely.
11pm-1am As sleep progresses, growth hormone levels peak to help restore the tissue that have been broken down during the day by stimulating the body to produce proteins and release energy from fats.
I hope the above outline helps understand some of the changes you feel during the day. It may also help you determine when is the best time to train or study/work. Some might use this as an excuse why they can never get to the gym. Train whenever you can make the time for it. Although you may do better if you can time it with your body’s natural peak time, it is far better to train when you are off peak than to not train at all.
