Dr. John Booker is a public health researcher at the University of Alaska Anchorage. It's very common for people suffering from winter depression to suffer from sleep disturbances, feeling sleepy during the daytime. They also tend to report that they're more irritable, that they have a hard time getting along with people, oftentimes are less interested in social activities or social interaction.
People often report weight gain during the wintertime.
Although the process is not well understood, researchers say Alaska's long dark winters disrupt the production of the hormones melatonin and ceratonin. Together these two hormones control our sleep-wake cycle. Basically think of it as melatonin being the chemical which makes you feel sleepy. So it tends to increase in the early mid evening period of time and then goes down in the morning. And ceratonin is sort of the opposite, it is the chemical which produces awakedness.