> > It didn't change "who I am", or artificially affect my personality, as I'd feared drugs would do; it simply restored a chemical balance that got upset with the lack of light and heat.
>
> Above and beyond Vitamin D deficiency? I know exposure to the sun gives you Vitamin D, and I know Vitamin D deficiency does put you in a sour mood, but I didn't know there were more serious problems that could result from a lack of exposure to sunlight. Would you mind, particularly, if I asked for details? You've piqued my curiosity. In any case, I'm glad things worked out.

Okay, I think I'll take a stab at Sam's question, since I have a personal fascination in all aspects of how light affects our sensibilities and reason, as well as how it interacts with the visual-reticular cortex. (That's the reason why my email addresses begin with the word 'optic', and indeed my older alias prior to my Wolfspirit namesake was once "New Lamps for Old". :-). So here goes, culled from various sources:


Think of SAD-related winter depression as five months of jet lag.

The human body has hundreds of 24-hour biological rhythms (called circadian rhythms), all controlled by an internal master clock. The sleep cycle is one such rhythm; body temperature is another. The brain's production of certain chemicals, like the night-time sleep-producing hormone "melatonin" is another.

In the summer, we wake up with the sun and go to bed when it gets dark, so our body's internal clock is kept in synch with the sun. But during the winter, most of us force ourselves to wake up while it's still dark. And that means that the sleep-wake circadium is out of step with our other rhythms, the latter of which cannot be controlled with an alarm clock.

Seasonal changes affect the light-sensitive pineal gland in the brain. When the days grow shorter, your eyes transmit less light energy along the optic chiasmatic nerves, and the pineal gland releases *more* of the hormone melatonin, which regulates night-time behaviour. In a cascading reaction, the increase in melatonin causes a *decrease* in internal body temperature. In a process similar to hibernation, this in turn produces sluggishness and sleepiness -- it makes you WANT to sleep when it's dark outside -- and in some people, continually struggling against the urge can make one irritable and depressed. Fortunately, bright light in the summer suppresses the melatonin release. And light has other effects on our well-being that are just as powerful as the regulation of the circadian rhythm. As Sam has already mentioned, sunlight mediates the production of Vitamin D for healthy bones. It also affects the age of onset for menarche in women, and in a strange and paradoxical genetic-repair mechanism, light could even be vital to repairing damage to our DNA.

The most crucial demonstration of the human biological response to light occurred in 1980, when Alfred Lewy, MD, with Wehr and other NIMH researchers showed that light suppresses secretion of human pineal melatonin (Science 1980; 210:1267-1269).