It is so hard to guess what goes on with l-dopa..my friend tends to take too much even of fava (even pod chips/ cookies / humus)and it gives her a sleep attack...so it could be the same with sinemet. Or, other than taking too much for the current need (which our needs change hr to hour according to activity level/ foods/ stress etc.... it could just relax the stiffness and tension and give a restful moment to just peacefully dose off?? Maybe try taking a smaller dose of l-dopa more often so there is not off times so severe during the daytime...watch diet. Eat lots of papaya if you do not have a latex alllergy...it can do so much for PD.
God Bless