The first tool is more body awareness. If we are actually trying to watch the breathing, then transfer your awareness to the body, and try to be more aware of the body. If you're leaning forward, hold it, feel it, and straighten up. This is the most basic way to work with sleepiness, to bring up more body awareness.
Sleepiness is very common in the first few days of the retreat, as we come from a busy life, then suddenly we are sitting down, and we may unconsciously feel, "Oh, it must be time to go to sleep"! It's all very relaxing. "I don't have to think about this, I don't have to think about that, I don't have to do this, I don't have to prepare that, I'm not doing anything! Therefore it's time for sleep!" So this tendency can come up when we do a retreat.
It was very interesting for us when we lived on our land and sometimes friends would visit us on the weekend. They lived very busy lives. For the first day they just slept, practically all day! We were out in the nice quiet bush land, no car noise, no hustle or bustle. It's often very busy in normal life. When we get into the forest, it's nice and calming to the senses. And when many sit down in quiet, we may want to be awake, but we go to sleep!
We have to use some appropriate techniques to bring up our awareness to the body, and also use energizing types of reflections, rather than tranquility type meditations. Tranquility often comes more in watching the breathing, and even Compassion/Lovingkindness can be tranquilizing.
So we have to bring up more energizing types of reflections. Confidence inspiring objects of meditation are energizing: reflection on the Buddha, the Dhamma, the Sangha, Sympathetic Joy. Reflection on death is very helpful for bringing up energy, but not all the time. And to understand our precious opportunity, How Fortunate You Are, these are energizing type of meditations, rather than tranquilizing. So we have to understand what are energizing and what are tranquilizing. Cause and Effect can be more tranquilizing, because it lessens the idea of "me", to just see Cause and Effect. Dukkha is very energizing. With all this Dukkha around, what do we want to go to sleep for? We want to be able to lessen it while we have got the opportunity.
There is actually quite a bit of balance in those energizing reflections. Not just death and Dukkha but also confidence inspiring objects. We do have to balance those energizing reflections as well.