A standard sleep questionnaire was given to the parents of 26 infants with protein-energy malnutrition who underwent polysomnographic evaluation. These investigations were repeated approximately 2 months after enrolment in a nutritional rehabilitation programme based on World Health Organization guidelines. Anthropometric values and serum serotonin levels were also measured. After nutritional rehabilitation there was a significantly higher percentage of non-rapid eye movement [‎REM]‎ sleep; 2nd REM time, and latency times for sleep and REM sleep increased. Percentages of REM sleep and serum serotonin levels decreased significantly. Protein-energy malnutrition seems to affect the sleep-wake cycle; disturbed serotonin levels may be among the factors responsible