The hormone, Melatonin, is one of the many factors involved in helping us to sleep. Computer screens usually emit a lot of light. Apparently the blue part of the spectrum is particularly ‘melatonin-suppressive’. Here is some free software that filters the blue light out of computer screens at dusk. I've just downloaded it and am running it and my screen is now less bright. It seems more restful for my eyes.
Note: Beta-blockers such as propranolol deplete the body of melatonin.