Purpose: Sleep and exercise influence each other through complex interactions. Sleep disorders incidence in athletes are high among several sports. Method: 24 CrossFit practitioners, of both genders, between the ages of 25 and 35, presenting difficulty in initiating sleep, with a total sleep time of less than 6 hours per night, were recruited into a double-blind, randomized, placebo-controlled trial that lasted 8 weeks. Participants received capsules containing 3 mg of melatonin + 1200 mg of GABA (gamma-Aminobutyric acid) or placebo at 10:00 pm daily. Sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI), cortisol and testosterone levels were measured and total-body skeletal muscle mass (TBSMM) by Lee’s equation performed by two different examiners. Results: Group melatonin + GABA gained an average of 600g of TBSMM while the control group lost 200g (p = 0.295). Both groups improve PSQI score (p = 0.007). No difference was found in cortisol and testosterone level. Conclusions: Although there was a difference between groups about TBSMM, it wasn’t statistically significant. Further studies will be necessary to demonstrate evidence of increased TBSMM from better sleep quality, with a larger number of participants and a longer trial.