Building your best day for healthy brain aging-The neuroprotective effects of optimal time use.

As the number of older people increases, so too does the prevalence of neurodegenerative disease. Worldwide, health organisations have identified the need for practical, affordable interventions to slow or delay the onset of neurodegenerative diseases such as dementia, for which there are multiple modifiable risk factors. The effects of various interventions on brain health has been investigated, including achieving sufficient physical activity, getting appropriate amounts and quality of sleep, and limiting sedentary behaviours. Few of these studies, though, have taken into account more than one lifestyle behaviour within a single study. Epidemiologists have recently initiated a paradigm shift to move away from studying the independent effects of each physical activity, sleep and sedentary behaviour, and towards an integrated 24-h time-use paradigm. Time is finite, and thus to increase time in one activity (for example physical activity), equal time must be taken away from other activities (sleep and sedentary behaviour). This 24-h time-use paradigm has begun to be used when studying obesity, adiposity and quality of life; however, to the authors' knowledge, it has not yet been adopted by cognitive neuroscientists for the study of cognition or brain function. This narrative review synthesises the evidence for the neurophysiological effects of physical activity, sleep and sedentary behaviour independently, with a particular focus on brain structure, function and neurodegenerative disease risk. Then, we conclude with a call to action, addressing the need for studies to move towards an integrated 24-h time-use paradigm.