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0:07Every year, people leave their doctor's
office with an unusual prescription.
0:12Instead of traditional treatments
like a pill, shot, or cream,
0:16they’re sent home with a recommendation
for light—
0:20or more specifically, light therapy.
0:23To understand how this works,
0:25let’s step out of the office
and into this valley.
0:28Here, as winter approaches,
0:30shorter days spark a cascade of changes
within the brains of its inhabitants.
0:35As a result, groundhogs retreat
to their burrows for hibernation.
0:39Weasels’ coats turn from brown to white,
and birds cease their singing.
0:44And scientists are finding
that seasonal behavioral changes
0:48similarly happen for some humans.
0:51Around 1% of people experience
seasonal affective disorder, or SAD,
0:56a mood disorder marked by episodes
of depression
0:59that recur at certain times
of the year.
1:01Most SAD cases appear during the fall and
winter months when days are shortest.
1:07And while many people experience mild
changes in energy or appetite
1:11SAD is much more serious.
1:14Its symptoms largely overlap
with non-seasonal depression,
1:17and can include pervasive
feelings of sadness,
1:20loss of interest in activities,
unshakable fatigue,
1:23dramatic changes in sleep and weight,
and suicidal thoughts.
1:27There’s still a lot we don’t
understand about SAD,
1:30but researchers have two main
theories on what may trigger it—
1:34both of which involve changes
to our body's daily rhythms,
1:37also known as circadian rhythms.
1:40When sunlight reaches our eyes,
1:42light sensitive cells in the retina send
a signal that travel along the optic nerve
1:46to our suprachiasmatic nucleus.
1:49This part of the brain is sometimes
referred to as our biological clock
1:54because it regulates our daily rhythms
of alertness, body temperature,
1:58hormone production, and cell growth.
2:00Each night after sunset,
2:02this biological clock
signals the release of melatonin,
2:05a hormone that triggers sleep.
2:07For most people, this melatonin release
is consistent
2:10regardless of the time of year.
2:12However, one theory posits that this isn’t
the case for some people with SAD.
2:17Instead, as the days get shorter,
2:19their biological clocks signal melatonin
to be released for longer each night.
2:24This kind of circadian change
isn’t entirely unheard of—
2:27weasels and groundhogs change their
appearance and behavior in winter
2:30due to a prolonged release of melatonin.
2:33But it’s unclear why
this would happen in certain humans.
2:36One idea is that, unlike most people,
2:39some individuals with SAD have retinas
that fail to adjust
2:42to be more sensitive to light in winter.
2:45A second hypothesis proposes
that SAD isn’t related
2:48to the duration of melatonin release,
2:51but rather to a complete shift in the
timing of circadian rhythms during winter.
2:56This could manifest as a phase-advance,
a fast-running biological clock,
3:00or a phase-delay,
a slow-running biological clock.
3:04A phase-delay could explain why
many people with SAD
3:07struggle to wake up in the morning.
3:09Essentially, their hormones are telling
them that it’s still nighttime.
3:13But since it’s unclear how any
of these circadian changes
3:16could cause such significant shifts
in mood,
3:19most researchers believe
other factors are at play.
3:22For example, as with
non-seasonal depression,
3:24people with SAD are more prone
to repetitive negative thinking
3:30They also tend to be more
pessimistic about wintertime.
3:33So much so, that some experts
believe living in cultures
3:36that embrace the winter season may offer
some protection against SAD.
3:40And while there is no single SAD gene,
3:42there are multiple genes that put you
at higher risk,
3:45which may be why SAD can run in families.
3:48Another mystery is that some people
with SAD
3:50experience the opposite
seasonal phenomenon:
3:53summertime depression.
3:55This may be triggered by too much light,
but perhaps also heat and humidity.
4:00Thankfully, despite these mysteries,
4:02doctors do know how
to help with this condition.
4:05Treatments used
for non-seasonal depression,
4:07like antidepressants and
cognitive-behavioral talk therapy,
4:10can improve SAD’s symptoms
and prevent future episodes.
4:14There's also light therapy,
4:16which typically involves spending
30 minutes or more each morning
4:19using a device that emits
full-spectrum light
4:22while filtering out harmful UV rays.
4:25Light therapy should be started under
the guidance of a healthcare expert,
4:28as it can be difficult
to figure out dosage and timing,
4:31and it can cause side effects.
4:33But with regular use,
4:35it’s thought to help shift
circadian rhythms back
4:37to their sunnier, summer-like position.
