The current study investigated how night-to-night variations in sleep duration relate to affective well-being the next morning as well as how the relationship varies for people of different ages. Using an Experience Sampling approach, 397 participants aged 12 to 88 years reported their sleep duration and their momentary affect on 9 mornings, on average. Associations between sleep duration during the previous night and morning affect differed depending on the participants’ age. For adolescents, for example, affective well-being in the morning was worse the shorter participants had slept the previous night. For adults aged over 20 years, however, affective well-being was worse following nights with shorter or longer than average sleep duration. This effect was more pronounced the older the participants were. The findings demonstrate that the importance of sleep duration for daily affective well-being is better understood when considering the age of the sleeper. In adults, but not adolescents, not only sleeping less but also sleeping more than one’s average can be associated with lower affective well-being.