Don't use a Pomodoro timer to study. Pomodoro timers are where you study for 15 minutes and take a 5 minute break or 25 minutes and then a 5 minute break. But what this does is it ends up leading to longer and longer breaks. Pomodoro timers are the worst thing for getting into studying flow. Once you've got into studying flow, you can stay concentrated and work for hours on end. Enforcing yourself to take a break every 25 minutes, every half an hour, every 50 minutes or whatever just completely interrupts your studying flow. Pomodoro timers are better used for tasks that you really don't feel like doing or when you just have to force yourself to get into the studying zone and you really don't want to. But after that, put the Pomodoro timers aside and just get into flow.