The Stanford Sleepiness Scale (SSS) is a self‐rating scale which is used to quantify progressive steps in sleepiness. The present study investigated whether the SSS cross‐validates with performance on mental tasks and whether the SSS demonstrates changes in sleepiness with sleep loss. Five college student Ss were given a brief test of memory and the Wilkinson Addition Test in 2 test sessions and The Wilkinson Vigilance Test in 2 other sessions spaced throughout a 16‐hr day for 6 days. Ss made SSS ratings every 15 min during their waking activities. On night 4, Ss underwent all night sleep deprivation. On all other nights, Ss were allowed only 8 hrs in bed. Mean SSS ratings correlated r= .68 with performance on the Wilkinson Tests. Discrete SSS ratings correlated r= .47 with performance on the memory test. Moreover, mean baseline SSS ratings were found to be significantly lower than corresponding ratings of the deprivation period.