The body has its own internal biological clock called the circadian rhythm that regulates physiology and behavior. The circadian rhythm is the 24 hour cycle that is the result of the earth rotating on its own axis. This causes the body to undergo dramatic and intense transformations within this 24 hour period. Jet-lag, working night shifts and Seasonal Affective Disorder (winter depression) are conditions that arise as a result of the body disconnecting from the universe.

The liver displays unquestionable circadian rhythmicity.
Kidneys are more active in the late afternoon.
Acidity of urine is greater at night than during the day.
Body cells divide during different times of the day.
Growth hormone is secreted shortly after people fall asleep.
75% of adrenal output (cortisol, adrenaline) occurs between 4 AM & 10 PM.
Humans produce greater amounts of heat during the day and conserve their heat at night.
Cortisol levels are higher in the morning, lowering your threshold for pain.
Rheumatoid arthritis peaks in the morning, osteoarthritis in the late afternoon.
Cancer cells are not connected with the natural circadian rhythm and divide all the time. Therefore cancer drugs must be given at specific times to avoid harming normal cells.

The human body oscillates between two major extremes- the waking state and the sleeping state. Each state of mind has its own chemical system that engineers the moment to moment physiology of the body. The waking state, which presumably you are in right now, is dominated by the aminergic chemical system (norepinephrine, dopamine and serotonin) and the sleeping state is usually associated with melatonin.

Caffeine and the Circadian Rhythm
Caffeine increases the activation of the mind-body state by stimulating the daytime (aminergic) chemical system. Excess caffeine consumption in the morning can disrupt the circadian rhythm by causing an overshoot of these energizing chemicals. The raw materials to produce the energetic state of caffeine are used up at a faster rate then they are replaced and this will tend to shorten the day. The concepts of the Circadian Diet suggest people drink their coffee or tea at 4 PM, a time when caffeine will tie in best with the biorhythms of the body.

Eating carbohydrates at night promotes relaxation by helping tryptophan cross the blood brain barrier. This increases the availability of the precursor to serotonin. The blood-brain barrier is a group of cells that separates the brain the general bloodstream. Amino acids normally compete with each other to cross the blood brain barrier because they use the same transport mechanisms. However, when carbohydrates are eaten along with the amino acid tryptophan (as on Thanksgiving Day when someone eats turkey and stuffing), the hormone insulin drives other amino acids into cells of the body and leaves tryptophan alone. With less competition across the blood brain barrier, tryptophan easily crosses and stimulates serotonin synthesis in the brain.

The constant oscillations of day and night entrain the biochemistry of the human body. People generally fall in love in the spring and get depressed in the winter when there are shorter daylight hours. In response to natural daylight, the brain’s pineal gland produces serotonin. For this reason people are advised to get at least 30 minutes of sunshine each day and purchase a natural sunlight lamp to use at your office desk. During the evening hours (without light), the pineal gland begins the chemical transformation of serotonin into melatonin. Melatonin is the hormone that helps you fall asleep and stay asleep. Melatonin production peaks at a specific interval during the night between the hours of 12 midnight and 3 AM. It is not advised to shock your eyes with light between these hours because visible light will knock out the production of melatonin. Use a dimly lit lamp if you get up during this time. One of the best ways to entrain your body with the universe is to establish a consistent sleeping pattern. You can restore biological rhythms by going to bed shortly after sunset and wake up with the sunrise.