Besides being the primary hormone involved in directing the functioning of the immune system, cortisol also dictates when we should be active and when we should rest. Cortisol levels follow a 24-hour cycle, peaking as the sun rises and tapering off as the sun sets. And as cortisol levels rise, we are given the energy to begin our day. As cortisol levels drop-reaching their lowest point about three hours after dark-our bodies enter into a period of rest and recovery, physical repair, and psychic regeneration.

Our immune system functions optimally if we go to sleep by 10 p.m. As we sleep, physical repair takes place between approximately 10 p.m. and 2 a.m.: Our immune cells patrol our bodies, eliminating cancer cells, bacteria, viruses, and other harmful agents. Then from about 2 a.m. to 6 a.m., we enter a stage of psychic regeneration. During this time, the brain releases chemicals that enhance our immune system. Throughout the night, we experience rapid eye movement (REM) sleep states and non-REM sleep, alternating between light sleep and deep dream states. This is how we process the mental and emotional events of the previous day and refresh our minds for the day ahead.

Most people need a minimum of seven or eight hours of sleep to accomplish all these tasks. Without sufficient sleep, the immune system is hard-pressed to keep up with its repair work. This creates the opportunity for disease processes to begin. Moreover, if cortisol is elevated at night-say, because you are anxious-these immune functions can become compromised, which ultimately leads to illness and disease.

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Have you ever wondered why your cold or flu symptoms get worse at night? It's because cortisol production regulates your immune system on a 24-hour cycle. As cortisol levels drop at night, your immune cells become highly active. The immune cells kill large numbers of bacteria and viruses, causing greater mucous production. As a result, you experience more congestion and coughing as your body attempts to get rid of the mucous.

At daybreak, when cortisol levels rise, the activity of the immune cells tapers off. The immune cells then reset and recondition themselves in preparation for the next nightly cycle.

Problems arise when cortisol levels are out of balance-say, when you fly to a different time zone-thereby compromising the cycle of immune function. As you can see, sleep/rest and recovery is essential for our bodies to repair and maintain optimum immune function.

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