Yes indeed, use of light exposure and/or melatonin singly or in combination are supposed to be of particular help in resetting sleep schedules... when the sleep schedule is "not right" because of circadian rhythm disturbances... as both will act to modify the circadian rhythm.

While light exposure is also useful to assist with depression, IMHO i don't see that it would affect someone's bedtime if the reason that is "off" compared to the rest of the world is not primarily due to an upset circadian rhythm to start with, but rooted in fear of sleeping in the dark, or of the darkness... etc... it might change the release times of melatonin, sure, but strongfear/anxiety can keep one awake even if one is actually tired.

Still light therapy can be useful for mood improvement, without necessarily tinkering with one's personal sleep habits - nocturnal or diurnal as they be. To this end, I would agree with what you suggest, Mari, that we could define "midday" in a relative way - meaning, several hours after one gets up. This would actually make sense, thinking about it, since the brightest light exposure would naturally occur around then, when the sun is high in the sky. Intuitively it seems to more "gentle" to me also than dowsing oneself with bright light upon waking... I could see where some depressed people would find that really jarring and upsetting, and rather than respond positively, land in that nomansland called a mixed state.

My thoughts.

~ waves ~