Systems and methods for distinguishing human users from bots

In one embodiment, a system is configured to identify, based on predetermined criteria, a first set of users of an online system who belong to a population segment. The system may monitor activities performed by the first set of users on the online system over a predetermined period of time and store the monitored activities as time-series data. A feature set associated with the first set of users may be generated by transforming the time-series data into a frequency domain. The system may train a machine-learning model using the feature set and other feature sets to determine whether activities associated with a given set of users exhibit diurnal behavior pattern. Using the trained machine-learning model, the system may determine whether activities performed by a second set of users on the online system exhibit diurnal behavior pattern.