How do they always know?

How do kids always know when you're exhausted and are trying to catch up on your sleep? I went to bed early(ish) last night, hoping to get a decent night's sleep for once.

Of course (because I was desperate for sleep), the little one was up ALL night crying, for no apparent reason! I'd get her quiet for 1/2 hour or so, and then it would start all over again. Now, I know I'm spoiled, since all 3 of my kids have slept through the night since they were tiny babies, but a couple times a year we have a rough night and a miserable, miserable day!

Luckily for me (after getting up WAY too early to take the big kids to school), she was still sleeping, so I managed to climb back into bed for another 2 hours of sleep. So at least I got 2 hours of sleep. But I feel like the walking dead... We'll try again tonight!