You used to be able to fall asleep anywhere. On a plane. On the sofa. Ten minutes after your head hit the pillow. Now you lie there, wide awake, wondering what happened.

If your sleep started to unravel somewhere in your 40s, you're not imagining it. And you're not alone. This is one of the most common things people in midlife search for, and one of the least well explained.

The usual answer is a vague "it's just part of getting older." Which is not only unhelpful, it's also not quite true.

You've been running on empty all day. By 9pm, you can barely keep your eyes open. You're yawning through your evening, counting down the minutes until you can finally get into bed.

And then you do. And nothing happens.