Deficiencies in thyroid hormone can cause someone to become quite depressed, and adding thyroid hormone can help depression whether someone is low in this hormone or not. Sleep and circadian rhythms seem to be tied to depression and mood disorders. Chronic sleep deprivation seems to worsen depression, and acute sleep deprivation can actually improve it. This decreased REM latency is one of the most reliable observations in depressed people. Problems with sleep regulation is a classic symptom of depression.
Circadian rhythms seem to be a key problem with depression occuring exclusively during the winter.
Genetics are significantly tied to depression. First degree relatives of a depressed person are roughly twice as likely to develop depression themselves. This correlation holds even when studies have looked at children of parents who are depressed are raised by adoptive parents without depression. Environmental stress has been linked to depression. Many people have observed that stressful life events often come before the first episode of depression, and less so before future episodes.