Following on from the last Rule, it’s hard to exaggerate how much brain space is taken up with everyday planning, logistics, mental to-do lists. As well as those jobs or projects that you’re consciously aware need a lot of planning (and that you’ll now be writing down in detail after reading the last Rule¹¹), there’s the rest of life in general.

However avid a list-maker you are, there’ll always be things you won’t write down. For example, you need to think through which order to go around the shops: you want to drop off the dry cleaning first so you don’t have to lug it around, and you want to do the food shopping last because there’s stuff you need to get back to the freezer quickly. But the post office will close at half past, and it’s quite a detour to the pharmacy to collect that prescription … You’re unlikely to plan that out on paper, but it’s still using up your brain.

Maybe you’re putting up shelves at the weekend. You’ll be thinking about what length screws to buy and where to get them – maybe you could do it when you collect your mum from the station? Although actually, perhaps if you went to a different store you could get the paint and the timber at the same time. But you’d have to do that before your mum turns up …

Phoning the bank, inviting people to a party, getting the kids ready for a new school term, changing electricity supplier, planning meals, updating your CV: life is full of things you need to think about. This all appears to sit invisibly alongside the rest of your life, except at times of significant overload, so it’s easy to underestimate how exhausting it is. Actually, however, it’s a big thing and the more of it there is, the more mentally exhausted you will be. To use a computer-based analogy, this is RAM – working memory – and the more information it has to hold, the less efficiently it will work.

Most of us are used to juggling these things around in our heads most of the time, but when we get overloaded they can cause enormous stress. You need to understand this, because then you’ll be able to work around it better. Clear those little tasks out of the way if you’ve got a busy time coming up – or save them for later – and cut yourself some slack. Recognise that if you’re getting to the culmination of a big project at work, it’s not reasonable to expect yourself to stay on top of myriad minor things at home too. Give yourself some empty-head time to help you cope (go to the movies, meditate, play computer games, have a cup of tea in the sunshine, play with the dog).

And always remember that this applies to other people too, especially your family. Don’t expect your kids to tidy their room at exam time¹² – if they take a break, it needs to be a proper break. This Rule, by the way, also explains why many traditional fathers don’t understand why mothers are so knackered; it’s not so much the physical effort of looking after the kids, it’s the mental exhaustion of keeping on top of everyone else’s diaries and logistical requirements as well as their own.