Once you create an account on Nightwatch, you will start receiving daily reports (Title: Your Daily Report) to your email address for Top 10 daily rank gains and Top 10 losses. If you wish to receive these reports less frequently or disable them, you can do that in the account settings.
All you need to do is:
Click on your profile icon in the top navigation tab,
Choose settings in the drop-down,
Change the interval to weekly, monthly, or never to altogether disable them: