Weight gain is related to light exposure.

Recent research conducted at Brisbane's own QUT has found that pre-schoolers exposed to more light earlier in the day tend to weigh more. Cassandra Pattinson, a PhD student and her colleagues (part of the Sleep in Early Childhood Research Group) studied 48 children aged three to five from six Brisbane childcare centres over a two week period, They found moderate intensity light exposure earlier in the day was associated with increased body mass index (BMI) whereas children who received their biggest dose of light in the afternoon had lower body mass index and were slimmer.

Interestingly physical activity was not associated with the body mass of the children but timing of sleep and light exposure was. This is the first time light has been shown to contribute to weight in children. With an estimated 42 million children around the globe under the age of five being classified as overweight or obese, this is a significant breakthrough. Modern advances in technology such as tablets, mobile phones, night lights, and television mean that our children are exposed to more environmental light than any previous generation and this has paralleled global increases in obesity. Clearly and as highlighted by QUT's  Sleep in Early Childhood Research Group, exposure of our children to different types of light at different times now needs to be part of the conversation when dealing with weight issues.

https://www.qut.edu.au/news/news?news-id=98439