Coffee doesn’t have to keep you awake to disrupt your sleep

For years, people have debated whether drinking coffee in the evening interferes with sleep. Some people can have caffeine late in the day and fall asleep easily, while others may remain awake for hours. But researchers are increasingly finding that simply asking whether coffee delays sleep may miss a more important issue: what caffeine does to the brain after sleep begins.

To investigate those effects, scientists are using EEG, or electroencephalography -- a method used to record the brain's electrical activity. EEG measurements can reveal much more than how long someone sleeps or how often they wake up. They can also provide clues about the biological quality and depth of that sleep.