Start taking a daily vitamin D supplement. Many people in our current age simply don't get enough sunlight or foods rich in vitamin D, and one of the symptoms is chronic fatigue as well as feeling a little blue. If it's exhaustion from keeping up with little ones, you might do your long meditations in the morning then save a few simple, short sessions for the evening. 10 mins of engaged practice is better than 30 minutes of falling asleep practice!
Also, including at least 9 prostrations while taking refuge at the start of each meditation session can help wake you up some
When I start falling asleep while meditating, I read that Khenchen Thrangu Rinpoche suggests visualizing a white lotus shining like the moon in the core of your chest sending white sparkles upward. Conversely if your mind is too lively, you can picture a black lotus facing downward sending dark light into the ground.
Or try looking up with your eyes only (keep your head level) and visualizing a bright white light above your head until the fatigue passes. Three deep, vigorous breaths can also help.
Worst case, open your eyes wide and look around in circles, slap your cheeks, switch to standing or walking meditation.
Awareness of sleepiness and its effects can be good, but only get you so far if your body is about to crap out on you!