Hericium erinaceus (HE), an edible mushroom, has been used as a herbal medicine in several Asian countries since ancient times. HE has potential as a medicine for the treatment and prevention of dementia, a disorder closely linked with circadian rhythm. This study investigated the effects of the intake of HE extracts on behavioral rhythm, photosensitivity of the circadian clock, and clock gene mRNA expression in the suprachiasmatic nucleus (SCN), a central clock, in mice. Although the HE ethanol extract only affected the offset time of activity, the HE water extract advanced the sleep-wake cycle without affecting the free-running period, photosensitivity, or the clock gene mRNA expression in SCN. In addition, both extracts decreased wakefulness around end of active phase. The findings of the present study suggest that HE may serve as a functional food in the prevention and treatment of Alzheimer's disease and delayed sleep phase syndrome.Hericium erinaceus (HE), an edible mushroom, has been used as a herbal medicine in several Asian countries since ancient times. HE has been reported to possess antitumor properties. Intraperitoneal administration of HE extracts to tumor-bearing mice for a 2-week period significantly reduced tumor size through the induction of NK cell activity, activation of macrophages, and inhibition of angiogenesis (11). Polysaccharides from HE also enhanced apoptosis induced by doxorubicin, a chemotherapeutic drug, in human hepatocellular carcinoma cells (13). Compounds isolated from the fruiting body of HE also reduced proliferation of HL-60 leukemia cells through induction of apoptosis (14). In addition, HE has been reported to have beneficial effects on the central nervous system, with HE intake found to reduce depression and anxiety in women (21). Hericenones isolated from the fruiting body and erinacines isolated from the mycelium of HE promoted nerve growth factor (NGF) biosynthesis in rodent astrocytes cultures (15). HE also prevented cognitive deficits induced by intracerebroventricular administration of amyloid β (25-35) peptide in mice (18). HE has been found to be effective in improving mild cognitive impairment in humans, which might be attributable to NGF synthesis promoting effect by hericenones (16). Repeated treatment with HE had neuroprotective effects in mice that were subjected to middle cerebral artery occlusion, through promotion of NGF synthesis (3). These studies suggest that HE has potential as a medicine for treatment and prevention of dementia. Dementia is closely linked with circadian clock, an internal clock that generates approximately 24 hours rhythm called as the circadian rhythm, and patients with dementia show functional circadian clock impairments (26). Although some studies have demonstrated that HE might be effective in the treatment and prevention of dementia, no previous studies have