A small, preliminary study has found that middle-aged men can do a brief high-intensity interval workout at night without suffering sleep disruption. Researchers from Charles Sturt University in Bathurst, Australia, conducted the study with 11 inactive men. The aim was to learn whether time of day—morning, afternoon or evening—would influence the effects of a 30-minute HIIT session on sleep levels and appetite.

Data analysis revealed little difference in sleep duration among the three exercise times. While no subjects perceived a variation in sleep or appetite, blood samples showed that evening exercise resulted in lower levels of the appetite-stimulating hormone ghrelin. The findings cannot be generalised to women or men of other ages.

“In the future, we hope to conduct similar studies recruiting women,” said lead study author Penelope Larsen, a PhD student at CSU School of Exercise Science, Health and Sport.

“Interestingly, power output during the sprint efforts was higher for the afternoon and evening trials compared to the morning trial, indicating that participants were able to perform better during latter parts of the day. Therefore, time of day may also need to be considered when planning training schedules.”

The study is available in Experimental Physiology (2019; doi:10.1113/EP087455).