In developing countries, youngsters start to work during the high school years. Several studies have shown the difficulties associated with double shift, i.e. to work and study concomitantly, and its negative health consequences. Work and study time, as social synchronizers, have significant effects on the sleep-wake cycle (SWC). The purpose of this study was to evaluate sleep patterns and sleepiness in young students before and after entering the workforce as apprentices or trainees. Participants were 40 adolescents (26 males), 15-18 years old (mean = 15.8 years old) engaged in a first-job program at a non-governmental organization (NGO) while attending evening high school in the outskirts of the city of São Paulo, Brazil. The participants wore actigraphs (Ambulatory Monitoring, Inc.) and registered subjective sleepiness on KSS (Karolinska Sleepiness Scale) along 7 consecutive days, before and after admission to the job. Descriptive analyses were performed, and the variables were tested by means of the t-test and repeated measures ANOVA taking factors day of the week and time of the day into consideration. The participants' sleep duration on weekdays exhibited significant difference before and after starting work (F = 4.55; p = 0.04); the mean sleep duration was 492 min (SD = 44 min) before admission to the job to decrease to 405 min (SD = 58 min) after starting work. The mid-sleep time exhibited significant difference on weekdays before and after starting work (04:57 h; SD = 45 min versus 03:30 h; SD = 54 min; F = 4.91; p = 0.03). Finally, also sleepiness on weekdays (F = 6.41; p = 0.04) and at the waking time (F = 10.75; p < 0.01) exhibited significant difference before and after admission to the job. This article emphasizes the fact that social synchronizers like working during the day and studying in the evening changed the participants' SWC and were associated with sleep restriction. Brazilian governmental incentives notwithstanding, simultaneous performance of several activities by young workers should be considered as an occupational health hazard. Employment policies targeting young workers should take the dual shift - study and work - and its effects on the sleep-wake cycle into account.