John didn't strictly-speaking need to piss just then, but he'd been tossing and turning on the sofa for half an hour. Time to get up and do something. At home, he'd have raided the fridge or turned on the telly. But Sarah slept so lightly that on his way back past her bedroom door he heard her mutter, thick-tongued with sleep, and roll over.
Well, Sherlock is spectacularly ignorant about some things...
And, John thought as he bundled up on the sofa again, it was just possible that Sherlock wasn't the only one.