Just to contradict myself, stepping over someone's legs is not really a dealbreaker. I have done so when necessary without feeling exceptionally imposed upon.

Of course you should still pick no-step-over seats if available at OLCI, but unless one of you is particularly short-legged or infirm, all the middle seats are fine for night flights, just as all the window seats are fine for night flights.

It is the aisle seats that are bad (for sleeping; for daytime use they are fine).