Modafinil’s half life is commonly quoted at 12 to 15 hours in healthy adults. That number describes how long it takes the body to clear half of the dose, not how long the subjective effect lasts. In practice, the noticeable wakefulness and focus peak around 2 to 4 hours after ingestion and taper gradually over the next 6 to 10 hours.
Food matters. Taking modafinil with a high fat meal can delay peak concentration by about an hour without necessarily reducing the total exposure. Caffeine, sleep debt and individual CYP enzyme activity also shift the curve. Poor sleepers often feel a stronger rebound fatigue once the drug wears off, which can be mistaken for a shorter duration.
Armodafinil tends to last longer because its R-enantiomer has a slower clearance. A 150 mg armodafinil dose is roughly comparable to 200 mg modafinil but can remain detectable in effect for 10 to 14 hours. Timing is therefore the single biggest lever for controlling duration. Early morning dosing protects nighttime sleep; afternoon dosing almost always cuts into rest.