What would be mechanism of action of sugar for a healthy individual? Blood glucose levels are kept in a pretty narrow band, so eating sugar generates insulin spike, and unless you just exercised and have depleted muscle glycogen storage it gets converted straight into fat. Insulin spikes also cause sleepiness.

Modafinil works extremely badly on me - it masks lack of sleep well enough, but it makes my mental performance extremely low, and makes me very irritable and unfriendly. Basically I get all side effects of sleep deprivation except I'm not aware of needing some sleep.

I have mixed experience with caffeine and amphetamine-like drugs. They seem to be useful for tiredness and focus enhancements to a degree.