You can use Natural Cycles even if you have an irregular sleeping schedule; the key is to find what works best for you!
We recommend that you try to measure five times a week to give the algorithm plenty of data to use and that you measure around the same time after the same amount of sleep - at least four consecutive hours of sleep. You can measure within a ±2 hour window from when you usually wake up and measure.
A tip is to always set the alarm at the same time, i.e., at 07.00 am and then fall back asleep if needed. If you find it challenging to find a regularity in when to measure, try to map out your sleeping schedule and find a good measuring time that fits you even if this means not adding five measurements a week.
It all comes down to finding what works best for you. It is best to measure and enter it as an "excluded temperature" than to not measure at all to maintain the habit of measuring. After each cycle, you can take a look at your graph to see if any temperature fluctuated and enter it as a "Exclude temperature" for those days.
If you're, for instance, changing time zones or changing from working day to a night shift, then you should enter your temperature as an "excluded temperature" the first morning, so your body has a chance to get used to the new schedule first.
If you never sleep for four consecutive hours, you can measure after the longest stretch of sleep that you usually get during the night.