Seasonal affective disorder (SAD) happens when the days are short (winter). It’s a depression tied to the increasing darkness or lower Sun in the sky. The pathophysiology is unknown, but it is thought that brain levels of serotonin and melatonin are implicated. Serotonin and melatonin are chemically similar, and derive from the same substrate. Serotonin is thought to be important in making people feel safe and comfortable, and it was once thought that low levels of serotonin cause depression (this theory is no longer accepted, but there is clearly a relationship between serotonin and depression.) Melatonin daily levels vary with daylight and the circadian rhythm. SAD demonstrated neuroendocrine influence and control over seasonal mood and physiology.
People with SAD have poor mood, low energy, carbohydrate craving, and difficulty concentrating. These are all symptoms of depression and of sleep deprivation. So is SAD a type of depression or of sleep disorder? Maybe both.
It is estimated that 6% of Americans get SAD. Women get it more than men and there is some tendency to run in families. People with non-SAD depression in their history and mood disorders in their familities are more vulnerable and generally have higher fluctuations with the seasons. People with unipolar depression are more vulnerable than people with bipolar.
People with SAD can suffer hypersomnia, insomnia, or both. Hypersomnia is much more common. There are good evolutionary explanations for this. The paucity if food in Winter should drive ancient humans to sleep more and consume fewer calories.
Brain studies show that when SAD strikes, the efficiency of sleep (percentage of time in bed spent asleep) decreases, and the amount of time spent in deep sleep decreases. The time in stage R (REM) actually increases.
Daily circadian rhythms are closely tied to the sleep-wake cycle, but SAD shows an example of another natural cycle that affects sleep. You might call these circannual rhythms. In some animals, circannual signals such as temperature and rain patterns can trigger physiological changes. The light levels and position of the Sun in the sky are among those proximal cues.
The severity of the depression is usually mild or moderate. Hospitalization is hardly ever needed for SAD. One out of ten SAD patients experience it in the summer, in what might be called reverse SAD.
Although humans are only mildly seasonal compared to many animals, our evolutionary lineage contains traces of instincts and behaviors related to the length of the daylight. This almost certainly plays a part in Seasonal Affective Disorder, which in this respect is similar to hibernation.
Doctors who get patients with SAD often prescribe anti-depressant drugs. Light therapy – exposure to a strong artificial light – works well for this type of depression. Bright white fluorescent bulbs are used. Sometimes just having the person spend more time outdoors helps too. Opening the shades and making the living space more sunny can be beneficial as is exercise.
However, it is known that the internal clock differs from the circadian clock. When scientists removed the suprachiasmatic nuclei from rats, the rats lose their circadian cycle timing, but they keep their circannual timing.