Efforts to establish an empirical basis for recommended sleep durations during adolescence need to take into account individual differences in optimum sleep, defined as the amount of sleep at which peak functioning is observed. A total of 419 adolescents (Mage = 15.03 years) with Mexican American backgrounds reported their nightly sleep duration and daily mood for a 2-week period at 1 or 2 waves of data collection, 1 year apart. Adolescents also completed an established measure of symptomatology. Multilevel modeling revealed a nonlinear association between sleep duration and next-day mood, whereby both too little and too much sleep were associated with elevated levels of daily distress. Significant individual differences in optimum sleep were observed such that younger adolescents and those with elevated levels of internalizing and total symptomatology evidenced greater sleep durations on nights before they reported their lowest levels of daily distress. Younger adolescents and those with higher internalizing and total symptomatology may need more sleep to reach their peak functioning the next day, at least in terms of daily mood.