The answer, surprisingly, is the latter. You're probably thinking that it doesn't make sense. There is a good explanation, though.

Foods that contain protein such as turkey, fish, or meat contain tryptophan, but they also contain an abundance of the other twenty-two amino acids. Hence, once ingested, a balance will exist in the amount of tryptophan in the blood compared to the other amino acids. When a meal is ingested consisting mostly of carbohydrate, the elevation in blood sugar levels stimulates the release of insulin. Insulin encourages long chained amino acids such as valine, isoleucine and leucine to leave the blood circulation and enter many tissues and organs (Wurtman, 1996). Hence, tryptophan is now left in a higher ratio in blood compared to other amino acids and can now be shuttled easier across the brain, eventually converting into serotonin.

To compare the influence of a pure carbohydrate meal versus a pure fat meal on fatigue, researchers from the Department of Human Nutrition, St. Bartholomew's and London Hospital School of Medicine in London, gathered sixteen volunteers, aged 20 to 47 years, from the staff and students at the college. Overnight fasted individuals were tested before and then at hourly intervals for four hours after a test meal of known composition. The three test meals all contained 400 calories of energy and their composition was as follows. Pure carbohydrate meal (CHO) consisted of a pure maltodextrin solution, pure fat meal (Fat) consisted of a long chain triglyceride emulsion, and the control meal (Control) contained 55 % of energy from carbohydrate, 15 % of energy as protein, and 30 % of energy as fat, using the same sources of carbohydrates and fat. Protein used was concentrated whey protein and soy lecithin.

The subjects eating the Control meal had a slight increase in fatigue within an hour, which stayed the same over the next few hours. Those eating a CHO meal had similar slight increase in fatigue within an hour, but the fatigue continued and got worse over the next few hours. Those eating the Fat meal did not feel fatigue for at least two hours, after which it gradually increased to equal that of the Control group. Therefore, those who ate the CHO meal had the highest amount of fatigue.

Biochemical blood testing showed the Control group to have a rise in total amino acids with a significant fall in the tryptophan: large neutral amino acid ratio (Trp:LNAA). The CHO meal caused a decrease in total plasma free amino acids and this was accompanied by a rise in the Trp:LNAA ratio. The higher Trp:LNAA ration would allow more tryptophan to cross the blood-brain barrier and be converted into serotonin. The consumption of the Fat meal, similar to the control meal, led to a fall in the Trp:LNAA ratio. The researchers don't have a good explanation for this.

It would be an oversimplification to think that fatigue induced after a meal is solely the result of changes of neurotransmitter levels. There are probably also changes that go on in the hormonal system. The fact that consumption of fat also induced slight fatigue without increasing Trp:LNAA ratio indicates that other factors are involved in fatigue inducement after a meal.

Comments
Over the long run, it is healthier to eat meals that are balanced for protein, fats, and carbohydrates. However, one can switch to a carbohydrate meal in the evening or a couple of hours before bed in order to induce sleep onset.

Wurtman RJ, Wurtman JJ. Brain serotonin, carbohydrate-craving, obesity and depression. Recent advances in Tryptophan Research, edited by Graziella Filippini et al. Plenum Press, New York, 1996.