There are three types of seasonal affective disorder (SAD), each of which is instigated by the change in seasons. I will outline each of them, but want to first describe the normal role of melatonin and serotonin in the sleep wake cycle.
Melatonin is the hormone that is primarily responsible for the feeling of sleepiness. It is released from the pineal gland in the absence of light and bathes the whole body. One of its most influential sites of action is the pituitary gland. Melatonin decreases the pituitary release of hormones responsible for activity, sexual function, and stress responses. The precursor for melatonin is the neurotransmitter serotonin. When the pineal gland is activated by lack of light, it starts converting serotonin to melatonin. The net result is a decrease in available serotonin. Because serotonin is typically responsible for our alertness during the day, this secondary effect contributes to sleepiness at night. The introduction of light in the morning causes melatonin production to decrease and for serotonin (and other hormones associated with wakefulness) to increase.
The change of seasons is sensed by organisms in a variety of ways, but one of the most influential for humans is the increase or decrease in day length. Although various factors likely contribute to variations of SAD, changes in daylight and a disruption of the melatonin/serotonin balance is key. Some of us, based on our genetic disposition, are very sensitive to the changes in daylight that occur with the change in seasons.
The most common form of SAD occurs in the fall/winter as the days get shorter (“winter blues”). People who suffer from this form of SAD simply feel like hibernating. They have trouble getting up in the morning, feel depressed, feel hopeless, have low energy, and may gain weight. One of the main theories for this type of SAD is that the decrease in sunlight shifts the melatonin/serotonin balance in such a way that there is more melatonin than serotonin. The increased exposure to melatonin results in a quieting of the pituitary gland, which results in the symptoms listed above. Because the pineal gland is creating all of the extra melatonin out of serotonin, less serotonin is available to support the efforts of happy wakefulness and concentration. Treatment for fall/winter SAD can include an increase in exposure to bright full spectrum light, exercise, and time spent outside during the day. Some medications can also beneficial.
Spring and summer SAD (“summer blues”) tends to have a slightly different effect on people but is also caused by a disruption of the serotonin/melatonin balance, although it is opposite. Symptoms include insomnia, irritability, increased libido, weight loss and anxiety.
Reverse seasonal affective disorder is not necessarily associated with the spring or fall and is defined by symptoms of elevated mood, excessive socialization, and hyperactivity. Some people think that this is a form of bipolar disorder. The fact that the change in seasons has such an uncharacteristic effect on some people is likely due to genetics, lifestyle, and stress. Their brains are hardwired to have this inverse reaction to the hormones monitoring sleepiness and wakefulness. The genetic anomaly may be in receptor subtypes or it may be in the processing of the secondary effects of serotonin.
As with all forms of SAD, if sleeping patterns can be normalized, the individual’s symptoms are likely to decrease. However, feelings of extreme hopeless or serious weight loss should be taken to a medical doctor. They will be able to best determine treatment for your specific condition based on a history of potential underlying causes.