This chapter reviews the manner in which sleep deprivation and circadian misalignment lead to impaired performance among healthcare providers. Several sleep and circadian factors affect the performance of physicians-in-training as well as other healthcare providers. In addition to acute sleep restriction, residents suffer chronic sleep restriction. Extended-duration work shifts also increase the likelihood of fatigue-related, self-reported medical errors. As learning is central to medical residency, any factor impairing resident learning is of concern. The only randomized evidence regarding elimination of 30-h shifts, and substitution with 16-h shifts, suggests that errors could in fact decrease by more than double this amount with widespread implementation of a 16-h shift limit. Lab- and field-based studies show us that extended resident shifts impair cognition and likely impair learning. Such impairment translates into error, poor patient care, and resident harm.