The circadian clock is an endogenous system designed to anticipate and adapt to daily changes in the environment. Alzheimer's disease (AD) is a progressive neurodegenerative disease, which is more prevalent in patients with type 2 diabetes mellitus (T2DM). However, the effects of circadian disruption on mental and physical health for T2DM patients are not yet fully understood, even though circadian disruption has been confirmed to promote the progression of AD in population. By housing db/db mice on a disrupted (a 6:18 light/dark cycle) circadian rhythm, we assessed the circadian gene expression, body weight, cognitive ability, and AD-related pathophysiology. Our results indicated that housing in these conditions led to disrupted diurnal circadian rhythms in the hippocampus of db/db mice and contributed to their weight gain. In the brain, the circadian-disrupted db/db mice showed a decreased cognitive ability and an increased hyperphosphorylation of tau protein, even though no difference was found in amyloid protein (A beta) plaque deposition. We also found that the hyperphosphorylated tau protein exhibited more disruptive daily oscillations in db/db mice hippocampus under the 6:18 light/dark cycle. Circadian alterations could promote the development of AD in T2DM.