Systems and methods are provided for using earphones with biometric sensors to identify and present information regarding performance periods. Fatigue level associated with fatigue experienced in response to a stimulus and recovery from such fatigue may be determined based on heart rate variability (HRV) data and learned user characteristics. One or more cycles of fatigue and recovery can be identified as a fitness cycle(s), each fitness cycle encompassing a period of time beginning with the stimulus associated with the fitness-related activity and progressing through recovery from the fatigue experienced in response to the stimulus associated with the fitness-related activity. A performance period may be predicted based on a pre-determined fatigue/recovery level instance within a fitness cycle.