People often think they slept more or less than they actually did, so when they fill out sleep surveys, their answers don’t match what machines measure — making those surveys less trustworthy for big health studies.

People who sleep too little (4 hours or less) or too much (more than 8 hours) tend to have a higher risk of dying from any cause or heart disease, while those who sleep 7 to 8 hours have the lowest risk—this might mean that how long you think you slept is more about your health problems or poor sleep quality than how well you actually rested.

People who sleep 5 hours or less each night are more likely to die sooner from any cause or from heart problems than those who sleep 7 to 8 hours, according to sleep measurements taken in labs.

People who sleep less than 7 hours a night are more likely to develop heart problems or die from them compared to those who sleep 7 to 9 hours, based on wearable device data.

People who sleep more than 9 hours a night don’t seem to have a higher chance of heart problems or dying from heart issues than those who sleep 7 to 9 hours, at least based on how their sleep was tracked with a wrist device.

People who say they sleep too little or too much are more likely to die from heart problems, but only those who actually sleep too little (measured by devices) show a clear link—so sleeping a lot might just mean you’re sick or your sleep is bad, not that you’re sleeping too much.