Radio doesn't have the same seasonal motivation as TV, which typically debut new shows around 'Fall Sweep' rating periods. Among other motivations, advertisers generally look to 'Fall Sweeps' with TV to decide what new shows to advertise on in the upcoming season. Local TV stations look to Fall Sweeps to use the new and hopefully well promoted network show line up to help increase lead-in audience to local news or other locally-based programming.
Radio on the other hand conducts ratings year around, but the season can affect the results. For example, Fall, Winter and Spring 'books', are sought after due to a generally higher response rate by adults. Whereas Summer to a lesser extent because of listener vacations outside the home market and a much lower response average. Generally stations who slant younger in their target demographics do better in the summer because youth are on summer break and available to participate in ratings.
In larger PPM rated radio markets, the closest you get to TV overnight ratings, are weekly ratings which are averaged to make up a single month total.