Long-haul, short-haul, makes no difference.
45 minutes on an eight hour flight, great.
5 minutes on a 55 minute flight, the last of four or whatever, also great.
I have done them all and if you need to sleep, sleep. Even 5 minutes will make a difference.
btw, I find that it is easier to nap when you are PF. The other guy is doing the paperwork and the 'phone so all you need to do is turn down your radio volume and kick back for a while.
Provided that you warn him/her to wake you if he/she feels tired or sleepy, all should be fine.