I've been like that this week, think it's still my body getting back on track after a nasty tummy bug sent a malabsorption condition loopy the other week. Think it was a mixture of that with back to work and the weather not helping, out in the bitter cold then in the warm, the warm then makes me feel more sleepy - nearly feel asleep in my book in a coffee shop after work yesterday :) Hope you feel better soon, I'm trying to sleep it out a bit, slept for 13 hours last night, had breakfast then went for another nap!
Give yourself and your body a break and try to get some nice you time, see if Mr K can help more with the kids (though I want my mum syndrome is difficult) so you can get a nice rest and a few good nights' sleep.