I'm going in March, and only staying two nights before leaving to visit a friend in the country. I'm arriving at 8:00 a.m. Saturday morning, and I'm sure I'll be too exhausted for a play that night, but would love to see something Sunday night. A cursory look at theatremonkey.com didn't turn up anything. Is there any theater on Sunday nights?

The theatres are dark Sunday nights. I just went to whatsonstage.com and browsed through several plays. Chitty Chitty Bang Bang has a Sunday matinee.

There are several Sat matinees, but you might not be up for those.

I would suggest that you try to catch a Saturday matinee if you really want to see a show. Most of them start at 2-3:30PM and last 2-3 hours (with intermission). You'll have a wide variety of choices since almost all of them have Saturday matinees. It'll keep you up so that you'll sleep at the "proper time" as well. Pick something lively (comedy, musical) since you'll still enjoy it if you're sleepy.
I usually do this before heading out of London and find that I get a really good night's sleep (and enjoy the play) and wake up adjusted to UK time.
In March (provided it's not Spring Break time) you'll probably find a lot through TKTS as well. You'll probably be too tired for an evening show.

There are very, very few theaters with shows on Sunday.

My only success with finding theatre in London on Sunday evenings has been in smaller fringe theatres in the hinterlands. But I've seen some mighty good stuff that way. Time Out is one good source for those listings.

Chitty Chitty Bang Bang, the Lion King, and a couple of others are the only west end shows w/ Sunday performances, all matinees. As Patrick says - some of fringe and suburban theatres do have Sunday shows.

Your best bets would be:

- a matinee on Sat (but if the weather is nice I'd hate to spend a jet lagged afternoon in a theatre - and that is major since I LOVE the theatre and go 4 to 6 times every week I'm in London),

- one of the few big shows that do have a sunday matinee,

- go to a concert instead. Many musical venues from clasical to pop to jazz do operate on Sundays.

topping for andib -- you posted the same question twice . . . . . .