Introduction: Prevention of childhood overweight should start as early as possible preferably in "the first 1000 days of life." Sleep is one of the modifiable health behaviors during this age period, besides dietary intake and physical activity. The aim of this systematic review is to summarize the existing literature regarding the association between sleep during infancy (age ≤24 months) and body composition measures during childhood (age ≤12 years). Methods: We registered the protocol of this systematic review (PROSPERO registration no. CRD42018087088) and conducted the review following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement. We searched for articles published until July 31, 2019 reporting on longitudinal associations with a minimal follow-up of 6 months. Methodological Quality was assessed and a narrative synthesis was performed. Results: We included 19 studies. Sleep was reported as sleep duration (n = 18) or sleep problems (n = 2). Sleep was assessed at least once before the age of 12 months in 14 out of the 19 studies. Methodological quality was rated as strong for five studies, moderate for five studies, and weak for nine studies. Conclusion: This narrative synthesis found inconsistent evidence that longer infant sleep duration during the first 2 years of life is associated with a healthier body composition during childhood.