Hmm? Say “Name”.

What? I’m just resting my eyes. Say “Name”. We need to get started.

Name: Social jetlag.

I’m going back to bed now. No you’re not. You have to ask about its appearance.

Look, I was up until 3am drinking advocaat and playing board games. Can’t this wait? No.

Appearance: Early January, for a lot of people. Almost constantly for an unlucky few.

What are we talking about here anyway? You know that feeling when you have to go back to work again after some time off?

I am familiar with it, yes. Well, it’s called social jetlag.

I thought it was just called “that feeling when you have to go back to work again”? Not sciencey enough, I’m afraid. This is a proper science thing.

Can you explain it to someone who is still mostly advocaat? I can try. Basically lots of your bodily processes – metabolism, sleep, alertness and so on – run according to a daily timetable controlled by the hours of sunlight. These are called your circadian rhythms.

I’m making myself an espresso. Would you like one? No thanks. Anyway, people’s rhythms vary quite a bit. Some are naturally late risers, some are naturally early, and most are somewhere in the middle. This is called your chronotype.

I think I’ll have a double espresso. Therefore, people with late chronotypes have to adapt their rhythms quite severely to suit their working hours. They manage this by using a lot of caffeine on work days, then have long lie-ins to recover. During a period off work, such as over Christmas, the lie-ins may even settle into a new routine.

So they revert to chronotype? Exactly. But then it takes a while to shift your rhythm back again, and during this period people become tired, irritable, clumsy, disorientated, ill and generally not much use. Hence: social jetlag.

I see. Some research even suggests that social jetlag is linked with smoking, drinking and depression.

So what’s the cure? Basically, you need to find a way of making sure that you never get a lie-in, even during the holidays.

Having children? Yes, that should do it.

Do say: “I think I’ll just move to a time zone that suits my new body clock.”

Don’t say: “Welcome to Greenland!”