For how we live.
In a two-week trial, 100 adults drank coffee or avoided caffeine on random days. Coffee did not raise the upper-chamber skipped beats it was blamed for — but it nudged up lower-chamber ones and cut sleep.
Guidelines settle on a limit near 200 mg a day — roughly two mugs of coffee. But the underlying studies are observational, and a vocal minority argues there is no clearly safe level.
The FDA's threshold traces to a Health Canada assessment and a 2017 systematic review of 381 studies. It describes a population, not a person, and the variation between people is large.