But on most days ... the advise is to get sleep sufficient to allow Zazen. It is best to sit in the mornings upon awakening, or at night just before bed. But you can pick another time when not so tired too. Then, take a bit of the sleepy zazen when it happens, if its too sleepy go get some sleep and sit on waking. If falling asleep during Zazen (although discouraged), just do that ... I promise not to beat you with my slipper (and hopefully it will not happen most days ... even Jundo falls asleep on the "sit-a-long" now and then, if you look closely! ) .
If you sit Zazen and it is ZZZZzzzzz, just perfectly ZZZZzzzz!
By the way, just adjusting the posture, opening the eyes a bit more and taking some breaths can help. I stretch my neck just a bit when tired during Zazen, and it seems to help ... as if a tiny string running from the top of my head to the ceiling were given a light tug. Or, one can return to following the breath for awhile. Monks in China and Japan have always had a close relationship to tea and caffeine (in moderation!). At more Sesshin I have attended in Japan or the West, tea and coffee (in moderation!) are always somewhere around.