The Circadian Cycle refers to a 24 hour time period where the behavior as well as physiology of organisms differs over time. The rhythm is determined by exposure to light and dark. When humans are exposed to light later in the day and during the night, from sources including cellphones, tablets, and indoor lighting, they are unable to begin the “inactivity” stage in the cycle. This negatively impacts their ability to rest and sleep. Lighting, particularly the color of lights, have significant impacts on individual’s moods as well. For example, red light has been seen to have the most minimal impact on the brain’s activity while white and blue light have a big influence on behavior/mood.
Source:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2717723/
https://blog.lsgc.com/how-lighting-affects-mood-circadian-rhythm/
http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1000145