We already know that the stimulating powers of caffeine make it an unsuitable choice for a late-night drink – at least if you want to get any shut-eye. But a new study adds a whole extra level of detail to our understanding of caffeine's impact on the brain during sleep.

Caffeine was shown to increase brain signal complexity, and shift the brain closer to a state of 'criticality', in tests run by researchers from the University of Montreal in Canada. This criticality refers to the brain being balanced between structure and flexibility, thought to be the most efficient state for processing information, learning, and making decisions.

However, this state might prevent restful sleep, the researchers suggest. The caffeine isn't just keeping us alert , but actually changing how the brain

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