The objective of this longitudinal, observational study was to verify whether a favorable change in sleep duration over 6 years could impact objective indicators of adiposity in adults aged 18-64 years. Short-duration sleepers (≤6 h per day; n=43) at baseline were divided into two groups: (i) those who increased their sleep duration to a 'healthy' length of 7-8 h per day at year 6 (mean increase: 1.52±0.66 h per day; n=23); and (ii) those who maintained their short sleep duration habits (mean change: -0.11±0.38 h per day; n=20). Adult individuals who reported sleeping 7-8 h per day at both baseline and year 6 (n=173) were used as a control group. Change in adiposity indicators for each sleep-duration group was compared by analysis of covariance. We observed that the two short-sleep-duration groups had similar baseline characteristics. However, short-duration sleepers who maintained their short sleep duration experienced a greater increase in body mass index (BMI) (difference: 1.1±0.36 kg m(-2), P<0.05) and fat mass (difference: 2.4±0.64 kg, P<0.05) over the 6-year follow-up period than short-duration sleepers who increased their sleep duration, even after adjustment for relevant covariates. We did not observe any significant difference in adiposity changes between the control group and short-duration sleepers who increased their sleep duration. This study suggests for the first time that shifting sleep duration from a short to a healthier length is associated with an attenuation of fat mass gain.