Hey alex from today and yesterday.
i don't officially practice vipassana,
although i guess the meditation i practice is very similar or perhaps exactly similar,
albeit explained differently (see the hindu anatta thread if you want to know more)
also i am not a teacher of any kind and i am not enlightened so make sure to not blindly accept my words as truth.
either way,
when i'm meditating i just can't sleep!
even if the body has very little energy left, as long as i am meditative then i can't become tired. (although i have never stayed awake for a whole night so maybe after 5 hours of laying awake i would eventually fall to sleep anyway)
so for me, i need to stop being meditative and let myself sink into ego/thought patterns,
if i want to be able to fall asleep.
when you feel the body is low in energy,
and you give in to feeling sleepy then that feels pleasurous.
how i see it, is that to feel sleepy is attachment to pleasure.
i guess the way i avoid "feeling sleepy" (and therefor avoid drifting off in tiredness

)
is to witness the sensations in the body without identifying with them.
for example:
- if the body is low in energy... and i don't identify myself as "i am the body",
then i do not feel sleepy... only the body is low in energy.
- if the body is giving the hunger signal... and i don't identify myself as "i am the body" then i do not feel hungry... only the body is giving hunger signal.
- if the body is giving the pain signal... and i don't identify myself as "i am the body" then i do not suffer... only the body is giving pain signal.
....
if you ask me, i guess your problem originates in identification with the body
as far as i know, vipassana is concerned with witnessing sensations and witnessing only.
so no attaching to sensations or pushing away sensations,
which means that you shouldn't assume yourself to be the body;
but instead that you must remain anchored as the witness.
when you assume yourself to be the body,
you crave pleasure and have aversion to pain.
i guess wanting to give in to sleepiness is craving pleasure
and perhaps simultaneously having aversion to the body feeling low in energy and muscles feeling a bit tired.
so i guess feeling sleepy is a mixture of "craving" and "aversion".