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 a lot 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 difficult 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 5 measurements a week.
It all comes down to finding what works best for you, but it is better to measure and enter it as a "deviating temperature" than to not measure at all in order to maintain the habit of measuring. After each cycle, you can take a look at your graph to see if any temperatures fluctuate and enter it as a "deviating temperature" for those days.
If you're, for instance, changing time zone or changing from working day to night shift, then you should enter your temperature as a "deviating temperature" the first morning so your body has a chance to get used to the new schedule first.
If you never sleep for 4 consecutive hours, you can measure after the longest stretch of sleep that you usually get during the night.