Late autumn is when the symptoms of seasonal depression, also known as seasonal affective disorder (SAD), typically start to set in. In fact, the main difference between the two is the fact that SAD is often brought on by the changing of seasons, per the National Institute of Mental Health (NIMH), a division of the National Institutes of Health (NIH). In addition, SAD is also more common in women; young people (it often starts in young adulthood); people with depression or bipolar disorder, especially bipolar II; and people who have relatives with mental illnesses.