What exactly is "chronobiology"?
Chronobiology is a science that studies how the clock inside the brain interacts with time cues in the environment.
The daily alternation of light and darkness is the main cue that keeps our inner clock in synch with the outside world. When such cues are absent or weak, our circadian rhythms of body temperature, hormones, sleep and alertness -- and yes, even mood -- get out of adjustment with earth's rotation about the sun.
Methods used in clinical chronobiology include timed exposure to bright artificial light to reduce insomnia or sleep-wake disturbances due to shift work, and the use of a spring-like lighting environment to fight winter depression. New applications include light therapy for chronic depression and depression during pregnancy.
The underlying idea is that even when there is plenty of light outdoors, our urban lifestyle tends to keep us in the dark, relatively speaking, making us vulnerable to mood slumps.