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The physical side of what occurs when we sleep is comparatively easy to understand. Although the specifics of what occurs at a molecular level is not fully known, the general physiological circumstances are understood. Our bodies slow down or suspend their aware-state responsive activities and then boost their maintenance and repair functions. Absorption of needed resources from the digestive tract is increased, tensions from active life are relaxed, the sifting of detrimental molecules by the kidney and liver is heightened and the immune system operates at a higher efficiency to deal with health problems, whether from an internal or external cause. Essentially our homeostasis, the body's balance, is restored from the disruptions that naturally occur during the active portions of our day.

While awake we undergo perceived stressors, consciously or sub-consciously. Such stressors induce our bodies to produce corticosteroids to maintain them in a state of preparedness to face whatever challenges may occur. Acute stressors, such as a fright, stimulate hormones such as epinethrine, more commonly known as adrenaline, to surge dramatically. This is a natural reaction to prepare us to either flee or fight the cause of the stress. In modern society, neither response tends to actually be appropriate in many situations, but the body still automatically responds in this way. Even when the stresses of our life are not this up front, we still produce considerable amounts of these hormones, keeping us in a state of preparedness.

When we fall asleep, the production of these hormones reduces significantly. We are now relaxed and our body recognizes that we no longer need to be prepared to run away or fight some indeterminate threat. Besides preparing us for such, corticosteroids also have a suppressive effect on two of our bodily systems; the digestive and immune systems. Therefore, once we are asleep and the amount of these hormones in our blood has reduced, our digestive and immune systems actually increase their activity by a considerable amount.

This is also why, when there is a need for either of these functions to increase their activity, we find ourselves feeling tired. If we have just eaten a large meal we feel the desire for a nap. This is probably even better displayed in large carnivores. Nature films often show large predators such as lions or wolves sleeping around the remains of a prey animal they have gorged upon. They have expended considerable energy

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