Light has an effect on our brain chemistry.  It regulates our circadian rhythm which is our body’s internal clock.  That clock controls a lot of functions in our bodies like SLEEP, libido, energy, mood and many other functions.  It tells the body when to wake up and when to go to bed.   Many many years ago people used to work outside on farms and in fields and they got on average about 10 hours of daylight per day.   A couple hundred years ago or so that has changed and more and more people started to work indoors and they are not getting the light they need.  Blue light from a bright sunny day is very healthy for the body and mind.  The timing of when you get the light exposure is very important.   Morning light stops the production of melatonin (the hormone that makes you sleepy) and triggers the flow of serotonin (the wake up hormone).  That is why you do not want to have your cell phone, tablet, laptop and other blue bright light emitting electronic devices around you and in use while you are trying to go to sleep.  Put them away about 2 hours before you want to have your head hit the pillow and fall asleep.  It disturbs your melatonin production.