Many people love to start their day with a cup of coffee or tea because that jolt of caffeine helps get their body moving. But a new study shows that same cup in the evening might not be a good idea. Researchers at the University of Montreal found caffeine puts the brain in a state that is more alert and could prevent it from recovering properly.

Caffeine is not only in coffee and tea, it is found in chocolate, energy drinks and many soft drinks. It is common in the American diet and one of the most popular psychoactive substances in the world.

Researchers used electroencephalography (EEG) scans to measure the effect of caffeine on the brain in hopes of determining what role it plays in the sleep cycle. The results showed caffeine increases the complexity of brain signals and enhances what is called criticality during sleep.

"Criticality describes a state of the brain that is balanced between order and chaos," said researcher Karim Jerbi. "It's like an orchestra: too quiet and nothing happens, too chaotic and there's cacophony. Criticality is the happy medium where brain activity is both organized and flexible. In this state, the brain functions optimally: It can process information efficiently, adapt quickly, learn and make decisions with agility."

Scientist Julie Carrier added, "Caffeine stimulates the brain and pushes it into a state of criticality, where it is more awake, alert and reactive. While this is useful during the day for concentration, this state could interfere with rest at night; the brain would neither relax nor recover properly."

The study involved 40 healthy adults ranging in age from 20-58. Their nocturnal brain activity was recorded on two separate nights - once when they were given caffeine before bedtime and again when they were given a placebo.

"We used advanced statistical analysis and artificial intelligence to identify subtle changes in neuronal activity," said study author Philipp Thölke. "The results showed that caffeine increased the complexity of brain signals, reflecting more dynamic and less predictable neuronal activity, especially during the non-rapid eye movement (NREM) phase of sleep that's crucial for memory consolidation and cognitive recovery."

On the night when they consumed caffeine, the brains of the participants showed changes in the electrical rhythms. Instead of slowing down and getting into theta and alpha waves, which are characteristic of deep, restorative sleep, they stimulated beta wave activity, which is more common during wakefulness and mental engagement.

"These changes suggest that even during sleep, the brain remains in a more activated, less restorative state under the influence of caffeine," Jerbi said. "This change in the brain's rhythmic activity may help explain why caffeine affects the efficiency with which the brain recovers during the night, with potential consequences for memory processing."

Click here to read more in the journal Communications Biology.