The differential effects of ageing on polysomnographic and EEG spectral characteristics of sleep were explored in men and women between the ages of 20 and 40. Men and women in their twenties were found to have similar percentages of slow-wave sleep (SWS) (% Stage 3 and 4) and mean EEG slow wave activity (quantified by spectral analysis). Significant reductions in the percentage of SWS and mean slow wave activity over the night occurred in men during their thirties but not in the women. This suggests that gender difference in SWS may emerge between age 30 and 40 in young adults. Men in this sample were also found to have significant increases in Stage 2 sleep, and decreases in REM sleep time, REM activity, REM density and REM intensity. No significant effects of age were found for women in any visually scored sleep variables. Both men and women had age related reductions in spectral power in the spindle frequencies. Taken together, these findings suggest that the sleep of men and women over age 20–40 may age differently.