Gil Fronsdal has a discussion on his website of sleepiness and lethargy, which he refers to as sloth and torpor. He looks at some of the ways that low energy states can arise and discusses several ways of investigating it and relating to it. You can find his article here: The Hindrance of Sloth and Torpor
Sharon Salzberg discusses several ways of dealing with sleepiness in meditation with kindness and compassion. As she and the Buddha suggest, at some point, we may just need to take a nap. You can find her article here: Advice for Dealing with Sleepiness in Meditation