The impact of sleep duration on psychological well-being was examined in 51,699 New Zealand adults using data from the New Zealand Attitudes and Values Study questionnaire in 2014–2016. Optimal sleep duration was reported by 58% of respondents to be 7 to <9 hours, but actual sleep duration was <7 hours in 37% and ≥9 hours in 4.5%. Short sleep duration was associated with negative psychological well-being, while long sleep duration was associated with an increased likelihood of depression. Rates of short sleep duration were particularly high in Māori and Pasifika.