Google-Fitbit refers to that chart as a "Sleep Pattern" chart. You will get the sleep pattern chart when the Sleep algorithm is unable to determine what your sleep stages (REM, deep, etc.) were, which happens for many reasons and to some people more than others.
When you get the sleep pattern chart instead of the sleep stages chart, you typically don't get Breathing Rate or Heart Rate Variation heart metrics returned either, which makes the whole concept of getting a Sleep Pattern chart pretty useless.
It is also good to know that paying for a Premium subscription is not required to get the Sleep Stage chart and the 5 Heart Metrics returned on a daily basis.
