Major depression during pregnancy or shortly after birth can be a devastating illness for mothers, impair the neurocognitive and socio-emotional development of the child and increase the risk of mental and medical disorders in the offspring later in life. Many people with mood disorders have disturbed body clocks. Oestrogen and progesterone are known to affect people's body clocks and it could be that changes in the levels of these substances in and around pregnancy disrupt people's body clocks and contribute towards depression. A substance called melatonin, which is secreted at night time, plays an important role in governing people's body clocks. Researchers from the University of California, San Diego, measured melatonin levels in pregnant women and new mothers with and without major depression. They found that melatonin levels were significantly lower in pregnant women with major depression but significantly higher in new mothers with major depression.
Parry, Barbara L. ... [et al] - Plasma melatonin circadian rhythym disturbances during pregnancy and postpartum in depressed women and women with personal or family histories of depression American Journal of Psychiatry 165(12), 1551-1558