You probably know this now but the smartphones release a blue light that makes you read clearly on the screen even in the sunniest days.
That light Is not adjusting to the day.. It is emitted all the time by laptops, TV and other gadgets too. This light imitates the sun and makes the brain confused to think it is day outside even when it is night. So, the brain stops to release melatonin and you have a hard time to sleep. So experts advise to remove every electronic or stop using them few hours before bed.