Although tuna has nothing in it that could make you sleepy, the digestive process which occurs when we eat food can make someone tired because it uses a certain amount of energy from our bodies.

Tuna contains omega-3 fatty acids which are particularly good for you and they are considered to be a healthy type of fat, unlike triglycerides which are found in the bloodstream and can contribute to heart disease along with other cardiovascular disorders.
Omega-3 fatty acids are also very important for certain brain functions as well as storing additional carbohydrates and glycogen as opposed to useless fat.