I got my hopes up when I sold 15 copies of my app on a good day, only to feel completely devasted when I sold only 4 copies the next day.
After some time I stopped looking at daily sales numbers, and switched to weekly numbers instead. But even that was too often.
Now I fetch my numbers roughly every other week, and don‘t worry at all about an individual numbers. Only by looking at trends over a longer timeframe can you make sensible decisions... The numbers just fluctuate too much from one week to the next.