"What time is it?"

At some point in most night shifts, you look at the clock. Often it's the first thing you do. It seems a harmless thing to do. I think it's one of the main things that keeps the shift going, and it's one of the easiest to change.

Think about what the clock gives you. It tells you that it's seven minutes past three. There's nothing you can do with that fact. You aren't going to get up. The alarm will go when it goes. The number is of no practical use to you.

What it does do is start the arithmetic. "It's seven minutes past three. If I get to sleep in the next quarter of an hour I'll have three and a half hours. That's not enough. I've got the budget meeting at ten. I'll be useless." Twenty minutes later you look again. "Half past three. Three hours twenty now, at best." Each look gives your mind a fresh piece of bad news and a fresh sum to do. It also adds a new item to the night's agenda, the problem of not sleeping, and this one gets worse every time you look.

The sleep arithmetic is a small night shift in itself. It has the same features. It feels like useful monitoring, it produces nothing, it runs on pessimistic guesses, and it makes you more alert. It comes with its own verdict, "tomorrow is ruined", which is as unreliable as the others. Most people get through the day after a bad night better than they predicted at four in the morning. They're tired and less cheerful, but they function. The forecast of total collapse is another product of the three o'clock mind.

Nicole Tang, Allison Harvey and a colleague published a study of clock-watching in insomnia under the title "Sleeping with the enemy". I haven't been able to read the paper or a summary of it, so I can't report its results. I mention it so you know that researchers take the idea seriously. The NHS advice doesn't hedge. Its guidance on sleeping better recommends keeping clocks out of view.

So turn the clock to face the wall, or put it in a drawer. If it's your alarm, it'll still go off. You don't need to know the time at any point between going to bed and hearing it. If you wake and it's dark, the answer to "what time is it?" is "night", and that's all you need to know. Some people find this remarkably freeing. Without the number there's no arithmetic. Without the arithmetic, a waking is just a waking, of unknown length, at an unknown hour, and it's much easier to drift back from.

The phone is a bigger version of the same problem. It's a clock, so it starts the arithmetic. It's a light, and bright light at night tells your body that it's day. It's also a door to every one of your problems. Your work email is in there, along with your bank balance, the news, and the message from your sister that you haven't answered. People pick it up in the night "just to check the time" and forty minutes later they're reading about interest rates. The night shift could hardly have a better tool, because the phone supplies new material faster than your own mind can.

The remedy is dull and it works. Charge the phone somewhere you can't reach from bed, and preferably outside the bedroom. If it's your alarm, a basic alarm clock costs a few pounds, and it can face the wall. If you need to be reachable at night for an elderly parent or a teenager who's out, turn the ringer up and leave the phone across the room. You'll hear it ring from there. You won't be able to scroll on it.