Z is for...

... zzzzz.

I know the feeling: I could sleep on a clothesline myself today.

"Could sleep on a clothes line" - tired enough to fall asleep anywhere.
This expression has its roots in the poverty of 19th century England amongst those who slept rough. For just two pence each, poor people could buy a night's lodging on the two-penny rope. This was a bench where these unfortunates would sleep sitting up, their bodies slumped over a clothes line stretched taut before them. The morning brought a rough awakening, for the landlord would often cut the rope to wake his impoverished guests before sending them on their way.