Chalk up one more benefit of sunlight—morning sunlight that is. A recent study from Northwestern Medicine demonstrates that timing and intensity of light correlate with body mass index (BMI). BMI is a numerical computation that compares height and weight, and it is considered a good measurement to assess obesity or the lack thereof. A high BMI usually means that a person is obese or at least approaching obesity. Optimal BMI is 18-25. Below 18 is underweight, above 25 is overweight, 30 is obese and 40 and above is morbidly obese.This research showed that exposure to bright morning light was directly related to BMI. After adjusting for confounders such as diet, exercise and sleep timing, it was determined that very early exposure to morning light correlated remarkably to lower BMI; even when light intensity was equal at different times of the day, those who received earliest bright light had lower BMI. In fact, for each hour later in the day that the light exposure occurred, BMI increased by 1.28 units. This fact is exceptionally important, since a person who has a BMI of 25 (upper ideal range) could approach 30, or obesity, simply by the habit of sunlight exposure later in the day, i.e. 10:00 AM rather than 6:00 AM.The authors of this research suggested that the mechanisms involved in weight control by early light exposure could be the following: (1) resetting the circadian rhythm (internal clock), (2) the greater amount of blue light in morning sunlight and (3) effects on melatonin production. Whatever the reasons, we now know that early-morning sunlight is important to weight control. It may also be important to other health issues.Since we know that the greatest vitamin D production, as a result of sunlight exposure, occurs around noon, I would suggest enjoying some early sunlight to begin the day, and then to engage in moderate, non-burning sunbathing around midday to optimize vitamin D levels. That should produce the greatest benefits possible.
 Reid KJ, Santostasi G, Baron KG, Wilson J, Kang J, Zee PC. Timing and intensity of light correlate with body weight in adults. PLoS One. 2014 Apr 2;9(4)