I take 100mg of modafinil when i don't sleep enough to stay wakeful throughout the day but 4 hours later the effects are gone. is it normal? should i try a full dose 200mg ?

Sleep apnea?: Have you been evaluated for sleep disorder ? If not, it is high time to do it before you resort to Modafinil.

Modafinil: Different people respond differently to the medication. 100 mg is on the lower end of the dosing range, and you’re right: it may not be enough for your needs - that may be the largest part of the problem. So when the drug starts to wear off, you may feel those effects faster than if the dose were higher. If you take the higher dose, be sure that it’s under the supervision of your physician.