AppleInsider.com — The iPhone 6 and 6s were released last month, but if you have an older iPhone, the best way to turn it on or turn it off is by going to Settings > General > About Phone > Turn on phone.
From there, go to General > Power Off and turn it back on.
From here, go back to Settings and turn the iPhone back on again.
If you are on iOS 11, you can also set the time to turn the phone on or go to the Settings app, then toggle on General > Accessibility > Accessible Controls > Accessory Accessibility.
If you have a Samsung phone, the phone can be turned off using Settings > Display > Display Control.