In order to help you more efficiently, it would be very helpful for us if you would tell us what version of macOS you are running and what version of WeatherCat you are using. We assume the latest versions of both, but just in case, please verify. Also, what type of data logger are you using (IP, USB, serial with adaptor, etc.). Thanks.
In the meantime, most of us allow the display to sleep, but not the hard drive.
Here is the reason why, from page 217 of the WeatherCat Manual:
Q: Is it OK to put my computer to sleep whilst running WeatherCat?
A: We do not recommend putting your computer to sleep if running WeatherCat. Instead we suggest either quitting WeatherCat before putting the computer to sleep, or shutting the computer down entirely. Alternatively, set the Energy Saver to sleep the display but never sleep the computer.