Your phone before bed: when to switch off.

You look at your phone before bed, like nearly everyone. Half ten, in bed, you open your inbox "just for a second". A client is disputing an invoice.

I'll reply tomorrow, you tell yourself. Then you draft the answer in your head, in the dark, for an hour. I hear this a lot from people I work with, and most blame the screen light. The light plays a part, but that email usually does more damage.

How to tell it's costing you sleep

• In bed, your mind switches back on. On the sofa your eyes were closing. A few minutes of screen and you're wide awake.
• The last look is never the last. Email, then the news, then messages.
• You may feel it the next day. Shorter temper, slower reactions, more of a sweet tooth.

Is it bad to use your phone before bed?

For your sleep, usually yes, for two reasons. The screen light tells your brain it's still daytime, and what you read puts it back to work.

Phone light looks like dawn. Some cells in your eyes aren't for seeing. They measure light and report to your body clock. Screen light is rich in blue, like early sun, and your body reads it as a wake-up call. Melatonin, the hormone that opens the door to sleep, can start later, by up to an hour and a half.

A work email is work. It demands attention and can raise your adrenaline and pulse, and anything that gets your heart going in the evening tends to hurt sleep. The news works the same way. Your brain reacts more strongly to bad news, and a distant crisis, read in bed, can set off the alarm as if it were on your street.

The most common mistake is trusting night mode. An orange screen takes out some of the blue, and that much helps. On its own it's rarely enough. The email hasn't changed.

When to put your phone away, and where

Screens off an hour before bed. Phone, tablet, laptop, and the series that has you gripped. Your body needs to change down through the gears. If an hour feels like a lot, start with half.

Dim the lights at home too. An ordinary ceiling light can be enough to hold melatonin back. In the last hour, turn off overhead lights and keep one low, warm lamp, like candlelight.

Your phone sleeps in another room. Charging on the landing, with the alarm already set. On the bedside table, temptation nearly always wins, and a screen lighting up in the night can disturb you. Your brain wants proper darkness. If you must stay reachable, leave only the ringer on and keep it away from the bed.

In its place, a paper book. In an armchair, under the low lamp, and read slowly. It helps your mind wind down. Go to bed when you feel sleepy, so bed stays the place you sleep.

Start by moving your phone out of the bedroom. It's usually the change that costs least and gives most. Keep it up for a fortnight. Ending the working day and what to do if sleep won't come each have their own article.