A study conducted in rats found that the half-life of adrafinil is about 4.95 hours[^1] However, in humans, the biological half-life is about 1 hour.
After one half-life, the amount of drug remaining in the body is 50%; after two half-lives, the amount is 25%, and so on.
The 1 hour half-life for adrafinil is misleading, however. The therapeutic effects of adrafinil are felt for much longer because adrafinil is converted to modafinil in vivo (in the body), and modafinil has a much longer half-life of 15 hours. Therefore, most or all of an ingested dose of adrafinil may be cleared from the body yet the therapeutic effects will persist because the metabolite modafinil takes longer to excrete.
The “functional half-life” of adrafinil (in terms of subjective effects – not in terms of the clearance of adrafinil from serum) is probably a little longer than the 15h half-life of modafinil. Because it takes time for adrafinil to be metabolized to modafinil, the onset of effects after ingesting a dose of adrafinil is delayed compared to modafinil. This prolonged time to reach peak plasma of modafinil after adrafinil ingestion indicates that the functional half-life adrafinil is slightly greater than 15 hours.
To learn more about modafinil vs adrafinil, check out my post comparing the two.
[^1]: Rao RN, Shinde DD, Talluri MV, Agawane SB. LC-ESI-MS determination and pharmacokinetics of adrafinil in rats. J Chromatogr B Analyt Technol Biomed Life Sci. 2008;873(1):119-23.] Link