best time of day to get sun

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Shine a Little Light
What you do: Get 20 to 30 minutes of bright light between 8 a.m. and noon

Why that's good: On top of helping sync your internal clock with the day's dark-light cycle, getting bright-light exposure in the a.m. hours (whether it's natural sunlight or indoor light) could help you maintain a healthy weight. In a small study published in PLOS One, researchers from Northwestern University found that the earlier subjects' light exposure occurred, the lower their BMI. The researchers speculate that light can help regulate metabolism, and that even 20 to 30 minutes of bright light between 8 a.m. and noon is enough to impact your weight.

One more thing: Morning light has the highest amount of short-wavelength blue light (which has the strongest effect on our circadian rhythms, helping regulate sleep and its influence on weight), which may explain why it has a more significant effect on BMI than afternoon or evening light.
As a reminder, always consult your doctor for medical advice and treatment before starting any program.