Although the sun has gotten a bad reputation due to ultra-violet radiation and skin cancers, the sun also has a host of beneficial effects, some that may even help increase your success at weight loss. Although the sun itself cannot directly cause you to lose weight, its effects on your body may provide a better atmosphere for you to be successful in your weight loss goals.
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If you live in areas of the world that experience dark and cold winters, you are probably familiar with seasonal affective disorder (SAD). SAD is a disorder that occurs when the lack of sunlight causes a drop in the neurotransmitter serotonin's levels in the brain. This drop results in mood changes such as depression. Not only can a drop in serotonin levels affect your mood, it may also affect other things such as your appetite and your feelings of hunger and satiation.
Hunger is largely controlled by a part of your brain called the hypothalamus. The hypothalamus works with serotonin to relieve hunger. As the body reaches its needed calorie intake, serotonin is released, causing a feeling of fullness. With a lack of sunlight resulting in seasonal affective disorder, and seasonal affective disorder causing a drop in serotonin levels, this drop can result in fullness not being achieved as quickly as when serotonin levels are higher. This can cause a greater calorie intake during times when the sunlight is not as prevalent.
Another way that sunlight may play a role in weight loss has to do with Brobeck's theory of heat-production. This theory states that as the body temperature drops, you feel hungrier. As the body temperature rises, you feel less hungry. This can result in greater calorie consumption during periods with less sunlight, which can cause a fluctuation in weight.
Another reason why sunlight may affect weight loss has to do with activity level. Although there are activities that can be done in the winter when sunlight is shortened, much of the population is more active during the spring and summer when the days are longer and warmer. Activities (whether hiking, camping, or going for a walk in the park) burn more calories. These added activities go a long ways to increase your overall calories burned in a week, leading to greater levels of weight loss.