Nightfall is a perfectly natural phenomenon, especially so if you're in your younger years and do not masturbate. Noticing it every night however is less common, but again, it doesn't automatically signal a problem. Do you tend to watch a lot of pornography, or did you have a very active sex life and then suddenly haven't had one for a long time? All these factors can play a role. However, this is likely nothing to worry about unless it's been a nightly occurence for too long. Your age and masturbation habits are big factors.