http://www.corante.com/brainwaves/archives/2005/09/15/when_are_you_most_alert.php

People reach their peak of alertness between 6pm and 7pm according to
Circadian Technologies. From an evolutionary perspective, this time of
day, early evening, was most likely spent securing the hearth for a
safe night's sleep. Human alertness also rises at dawn or early
morning. While a CMO magazine piece warns employers that trying to mess
with this natural cycle won't get them very far, emerging
neurotechnologies from companies like Cortex Pharmaceuticals are making
are making headway on improving alertness and attention.