# Cycles Key Idea: > Should you think about the *length of time* something has been in existence for (or any of similar proxy for age), or the number of *cycles* or *iterations* that it has undergone? * [Thermal Cycling Test: why perform it?](https://sinovoltaics.com/learning-center/testing/thermal-cycling-test-why-perform-it/) * page 90, The Energy Switch * Aircrafts → [How Long Do Airplanes Last?--Factors Affecting Their Lifespan | Knisley Welding](https://knisleyexhaust.com/blog/how-long-do-airplanes-last-factors-affecting-their-lifespan/) * [Heart Rate Variability (HRV): What It Is and How You Can Track It](https://my.clevelandclinic.org/health/symptoms/21773-heart-rate-variability-hrv) * [Cycles and the Cyclic Nature of Systems - SEBoK](https://sebokwiki.org/wiki/Cycles_and_the_Cyclic_Nature_of_Systems) * Cycling page 90 (HRV training) , glucose cycle causing cell aging, the cycle highlights that the key independent variable is not time, but rather whatever the cycle is highlighting (eg the pressure in an airplane, or torsion/force in solar cell. These aren’t time dependent, but rather cycle dependent. Gets at causes more effectively)