How accurate are sleep cycle calculators?

Sleep cycle calculators are only rough planning tools. Most assume that every cycle lasts approximately 90 minutes and that a person takes a standard amount of time to fall asleep. In reality, cycle lengths vary between individuals, throughout the night and according to age, stress, alcohol, medication and sleep deprivation. A calculator also cannot identify when you actually enter light, deep or REM sleep. Wearable devices may provide additional estimates using movement and heart rate, but they still cannot match the accuracy of a clinical sleep study that measures brain activity. A calculator may help you consider possible bedtimes, but it should not replace enough total sleep, consistent scheduling or medical evaluation when persistent sleep problems are present.