Bright light therapy is a method of maintaining or restoring the natural circadian rhythm by assisting daytime awakening using bright lights. Postoperative delirium is one of the potential complications encountered by patients receiving postoperative care in the intensive care unit (ICU), but there have been no studies on the use of light for the prevention of postoperative delirium. The objective of this study was to examine whether the circadian rhythms of patients after surgery for oesophageal cancer can be adjusted and whether the postoperative delirium crisis rate can be reduced by bright light therapy. The subjects were 11 patients operated on for oesophageal cancer in Osaka University Hospital. After informed consent was obtained, they were divided into a study group and a control group by a random sampling method. After removal of the endotracheal tube, the study group was exposed to light. The light intensity was about 5000lx immediately before the eyes, and the distance from the light source was about 100 cm. The control group was placed in a natural lighting environment after extubation. In both groups, the rhythms of physical activities and autonomic activities were monitored after surgery, and delirium was evaluated. A significant difference was observed in the delirium score between the study group and control group on the morning of day 3 of bright light therapy by the Mann-Whitney U-test (P=0.014). The study group could begin ambulation about 2 days earlier than the control group. Bright light therapy may reduce the rate of postoperative delirium and make early ambulation possible. However, our study involved a very small sample size. We want to increase the sample in the future after having reviewed clinical application methods.