Change what you eat
Some foods make you more sleepy than others.
Foods with sugar
or white flour (as opposed to whole grains) are often the problem.
Foods such as turkey which contain tryptophan
are well known to cause sleepiness
too. Other foods with lean protein
and low carbohydrate are more likely to keep you alert. Start keeping a log of what you eat and how sleepy you are afterwards.
Answer assisted by David Miller, Medical Student