While riding at a hard effort, the body utilized primarily glycolytic metabolic pathways for the production fo energy. The fuel for glycolytic energy production is carbohydrate. Carbohydrates are primarily stored in the body in the form of blood glucose, muscle glycogen and liver glycogen.
As you ride hard, the levels of these energy stores are depleated. After a two hour ride you are likely in a state to hypoglycemia, which means you have depleated your carbohydrate reserves to the point where your blood sugar is lower than normal. Sleepiness is one of many side effects of hypoglycemia.
The metabolic by-products of glycolytic energy production are acidic in nature. During intense training, acidic compouonds accumulate in the blood faster than they can be buffered out. Thus, intense training leads to a state of exercise-induced metabolic acidosis. Sleepiness is one of the many side effects of acidosis.
Acute hypoglycemic and acidotic states are two of the goals of endurance training. With proper nutrition and rest, the body recovers from these states with a larger capacity to resist them. In other words, training stresses the body and the body responds by becomming stronger.
To avoid the feeling of extreme sleepiness following a hard ride, nutrition is the key.
For the evening meal the night before the ride, eat a dinner with a lot of protein, a medium amount of complex carbohydrates, plenty of fat, vegetables, and water. For breakfast (about an hour before the ride) eat plenty of protein, fat, complex carbohydrates, water, and maybe some black coffee. Try to avoid eating sugary food with your meals.
Metabolism of protein and fat for energy is a much lengthier process than metabolism of carbohydrates. By eating a large evening meal, you will ensure that your glycogen levels are 'topped off' at the beginning of the ride. by eating a large breakfast, you will ensure that there are slow-burning nutrients in your body at the end of the ride that can be converted into energy. This will help to reduce the post-ride hypoglycemic slump.
During a two hour ride, stick with water for the first 90 minutes. For the last 30 minutes introduce a sports drink or gel for a little simple carbohydrate boost.
Immediately after the ride, eat some easy-to digest simple carbohydrates. Fruit is an excellent choice since it serves to normalize both blood glucose levels and blood pH. Yogurt and berries with plenty of water is awesome after a ride.
After the snack, take a shower and stretch. Then have a normal lunch and see how you feel.
Give this a try and let me know how it works for you.