The present study examined whether Lactobacillus casei strain Shirota (LcS) improves sleep quality under psychological stress. A double-blind, placebo-controlled trial was conducted in healthy 4 year medical students exposed to academic examination stress. The trial was repeated over two consecutive years in different groups of students, and the data were pooled. For 8 weeks prior to and 3 weeks after a national standardised examination, a total of 48 and 46 subjects received a daily dose of 100 ml of LcS-fermented milk or non-fermented placebo milk, respectively. Study measures included subjective anxiety, overnight single-channel electroencephalography (EEG) recordings, and the Oguri-Shirakawa-Azumi (OSA) sleep inventory scores of subjective sleep quality. Total OSA scores were significantly lower than baseline on the day before the exam and recovered after the exam, indicating a stress-induced decline in sleep quality. There was a significant positive effect of LcS treatment on OSA factors for sleepiness on rising and sleep length. Sleep latency measured by EEG lengthened as the exam approached in the placebo group but was significantly suppressed in the LcS group. The percentage of stage 3 non-REM (N3) sleep decreased in the placebo group as the exam approached, whereas it was maintained in the LcS group throughout the trial. Delta power during the first sleep cycle, measured as an index of sleep intensity, increased as the exam approached in the LcS group and was significantly higher than in the placebo group. These findings suggest that daily consumption of LcS may help to maintain sleep quality during a period of increasing stress. The observed retention of N3 sleep and increased delta power in the LcS group may have contributed to higher perceived sleep satisfaction.