Yep.... it is true and it is due to a chemical release that takes place in their body after they have a sexual release - they become sleepy / tired.
I guess this was built into them as to not ware us, the female, out... and so they will want to come back to their woman for more.

On another note... might I say this is the main reason we "the female" should not get mad at our man when he falls asleep on us after sex... even though we "the female" may want to talk and snuggle once we enter the after glow of sex - our man is thinking sleep sleep sleep.

(just another wonderful difference in the sexes)