Another thing you can do is take a really short nap in the afternoon with the clear intention when you lie down that you will arise at the end of the alotted time. You can use a timer too.
The Buddha gave some advice to Ven. Moggallana in AN 7.61 concerning drowsiness. What's below is from my notes and a short nap would fall under the last method.
AN 7.61: How to remove drowsiness, from most to least preferable way
o Don’t attend to or concentrate on the object that you were attending to when you became drowsy.
o Ponder, examine, and mentally inspect the Dhamma as you have heard it and learned it.
o Recite in detail the Dhamma as you have heard it and learned it.
o Pull both ears and rub your limbs with your hands.
o Get up, rub your eyes with water, survey all quarters, look up at the constellations and the stars.
o Attend to the perception of light; undertake the perception of day; as by day, so at night…as at night, so by day; develop a mind imbued with luminosity.
o Walk back and forth, perceiving what is behind you and what is in front, with your sense faculties drawn in and your mind collected.
o If all else fails, take a very short nap in the lion’s posture and get up quickly when done, thinking: ‘I will not be intent on the pleasure of rest, the pleasure of sloth, the pleasure of sleep.’