[Cthulhu_]

Well on the one hand it is brain activity - games and code excite the brain, and it can't simply click shut off and sleep.

On the other hand, it's light - the light from your computer screen goes straight into your eyes and to your brain, that tells you "IT'S STILL DAY". The brain / eyes need darkness / dusk to start producing melatonin, the sleep molecule.

So, turn off your screens, don't look at mobile phones, and if possible turn the TV off too, or at the very least dim them to the lowest brightness setting at least an hour before attempting to go to sleep, preferably two.

Software like f.lux will help too; studies show that white/blue lights will wake you up, while red, warm colors will make you sleepier. f.lux will make the screen a nice warm, soft red after sundown, mimicking natural sundown (while making it brighter / 'colder' during the day, keeping you awake while you work).