Two lab-based trials quantified the effect of a range of continuous light (LL) treatments overwinter on the somatic growth and incidence of sexual maturation in the fall age 2 of individually identified under-yearling Arctic charr (Labrador strain) at constant 10°C. Controls were maintained under simulated natural photoperiod (LDN; Latitude 45°N). In trial 1 (n=120/trt, mean 54g initial body weight), LL starting the 1st day of either October, November, December, January or February (all returned to LDN on April l) resulted in a step-wise reduction in maturity of 43, 55, 59, 67 73% compared with 77% in the control (sexes pooled). In trial 2 (n=90/trt, mean 24g initial body weight), LL starting either August 13 or October 1 and ending April 1 reduced maturity to <10%, compared to 33% (LL Dec1-Apr1) and 50% in the LDN controls. LL starting October 1 and ending either February 1, April 1 or June 1 were equally effective, resulting in <15% maturity. The importance of returning from LL to LDN in spring to reduce maturity was demonstrated by the LL constant (Oct–Oct) treatment which resulted in relatively high maturity, 31%. The reduction of maturity rate was independent of both somatic growth and condition factor during winter in all treatment groups. Among fish that matured, the timing of completion of sexual maturation was affected by both the LL start and end date. •24h light overwinter effectively reduced early maturation in diploid Arctic charr.•Gonadal maturation can be either stimulated or inhibited by 24h light dependent on both the start date and duration.•Photoperiod induced reduction of maturity rate was independent of somatic growth.