The short answer
The content matters more than the light. A phone at midnight is a problem because of what is on it, not mainly because of its colour temperature.
- Blue light does suppress melatonin, but a phone at normal brightness produces far less light than daylight and the effect is modest.
- What genuinely keeps people awake is what the device delivers: messages, news, arguments, work email and things designed to hold attention.
- A consistent wind-down matters more than the device: same time, low light, nothing demanding.
- Get bright light in the morning instead. That is the signal that sets the body clock, and it is the more effective lever.
What will ruin it
Anything that involves the alarm clock being on the phone means the phone is within reach at three in the morning. That is the real mechanism, and a cheap alarm clock solves it.
Why it works
The body clock is set mainly by bright morning light. Evening light shifts it somewhat, but attention and arousal are the larger effect for most people.
This is general information, not medical advice, and it does not replace a GP, a pharmacist or NHS 111. Call 999 in an emergency. Full disclaimer.