This November 6, most Americans will rejoice at the 25-hour day on Sunday, since they gain an hour of sleep. We'll "fall back" and set our clocks an hour behind at 2 a.m. to lighter mornings and darker evenings. The end of Daylight Saving Time (DST) may give us an hour of sleep, but it can also take away from our health.

We know humans and other mammals have an internal clock that regulates our sleep-wake cycles, among other things. Light provides cues that affect pupil dilation, alertness, melatonin levels, and heart rate. The shift in the amount of light and the fluctuation in seasons contribute to the psychological and physiological changes experienced by both humans and animals. For example, during the long winter days, Siberian hamsters' testes increase to almost 17 times their size during short days, when they receive the least amount of sunlight exposure. Humans tend to be more susceptible to mood disorders and depressive episodes, like seasonal affective disorder (SAD), related to both the seasons and change in time.

When it comes to DST, it can have both positive and negative effects on human health.

Moody

People with SAD, a mood disorder people experience in the darker winter months, are more likely to feel worse after the end of DST. A 2001 study published in Archives of General Psychiatry found people with SAD secreted the sleep hormone melatonin for longer periods during the winter nights than summer nights. Typically, melatonin is not affected by the change in seasons.

Depression

The brighter mornings and darker evenings could affect happiness levels in several people. A recent study published in Epidemiology found increased depressive episodes in the fall because the transition from summertime to winter leads people to leave work in the dark. The sudden change to the start of DST had no effect on depressive episodes in patients.

Cluster Headaches

The effects of DST on circadian rhythms can lead to the onset of debilitating chronic pain. Cluster headaches — severe headaches that recur over a period of several weeks, in which the pain is localized to one side of the head — seem to be triggered by changes in circadian rhythms, including the transition in and out of DST, according to the University of Maryland Medical Center. Most cluster headache attacks occur between early evening and early morning hours, with a peak time of midnight to 3 a.m.

Stroke

The rise of getting a stroke can increase by 8 percent in two days following a change in time, either in the fall or spring. Researchers speculate the change in circadian rhythm affects those who are already at risk for an ischemic stroke, which is caused by a blockage in blood flow to the brain. However, this effect weakened after two days, as the participants adapted to the change.

Fewer Heart Attacks

DST increases stroke risk, but decreases the incidence in heart attacks. A 2014 study found heart attack risk fell 21 percent later in the year, on the Tuesday after the clock was returned to standard time, and people got an extra hour's sleep. The researchers note a strong link between lack of sleep and heart attacks seen in previous studies. However, there’s still a lack of understanding why people are so sensitive to sleep-wake cycles.

More Sleep

Inevitably, an extra hour added to our day means we gain an hour of sleep — if we can actually sleep. Lack of sleep has become a health issue in America, with a third of adults not getting enough shut-eye, according to a study conducted by the Centers of Disease Control and Prevention. Research has shown lack of sleep is linked to a greater risk of obesity, diabetes, heart disease, mental illness and other chronic conditions.