Thank you for posting and that is a great question! In the reporting of each email campaign that is sent out we do record the time that each contact opens your email. Unfortunately there isn't currently a way to automatically track who is opening email campaigns during a certain time period. In order to find that information it will require that you export the reporting for each email campaign and manually review the times that your contacts are opening your email campaigns. I have included some helpful FAQs on exporting email reporting:
I will also relay this feedback to our engineers as a feature request. I hope this was helpful to you! Please let us know if you need any further assistance.