Maintenance of the circadian rhythm is important because it helps keep the body's functions running normally.
Alteration of the circadian rhythm to a significant degree may lead to mood disorders including mild depression and seasonal affective disorder.
Abrupt shifts in routine such as shift change or travel resulting in jet lag can alter the sleep cycle and also environmental factors such as decrease in number of daylight hours. This can negatively impact the secretion of melatonin which induces us to sleep which will lead to less sleep and thus irritability and mood swings.