From a physiological standpoint, morning wood--medically referred to as nocturnal penile tumescence (NPT)--is a normal, healthy phenomenon produced by the complex interaction of hormones, neurology and vascular health. Increased blood flow to the pelvic region is caused by the parasympathetic nervous system during REM sleep cycles, while testosterone levels peak at above-average rates in the early morning hours. This biological process acts as the body's self-fitness mechanism -- essentially 'working out' erectile tissue in order to keep vessels healthy and oxygenated. You can think of it as your body self-diagnosing on the sexual function front. In fact, loss of morning erections is sometimes an early warning signal of heart problems or hormone deficiency. There are two types of morning wood: nocturnal penile tumescence (NPT), which usually occurs during the REM stage of sleep and can happen up to five or six times a night (not all men can remember them, however), and good old "morning wood," which is blissfully post-printing. (men are basically like print using an ink on a roller). While people joke about morning wood, it's actually an important biomarker of men's health. The phenomenon is usually due to a flood of noradrenaline and dopamine during the last sleep stages, and weakened inhibiting signals from the brain prefrontal cortex after waking. It's nature's way of making sure the old "rocket" is firing on all cylinders -- studies have shown that men have 3-5 nighttime tumescence (NPT) events a night, the strongest generally occurring in the last REM cycle before we rouse for the day. These spontaneous erections are far from purely sexual; they are evidence of the amazing autonomic regulation of our bodies. But if frequency drops precipitously or ceases altogether -- especially in younger men -- it is worth mentioning to a physician to assess vascular, neurological or endocrine health.