You can understand how Smart Sending and Quiet Hours work and also how to change their period by going to Settings > Messages in the top right corner.

Smart Sending

Smart Sending prevents you from inadvertently sending messages too often. With Smart Sending, subscribers will be skipped from receiving a message if they’ve already received one within the period set.

Smart Sending Period

The default Smart Sending period is 8 hours. This means that subscribers who have received a message within 8 hours of your most recent send will be skipped.
You can change this period to either 8, 12, 24, 48 or 72 hours.

Quiet Hours

Quiet Hours prevent your subscribers from receiving automated messages during specified hours (like the middle of the night). If an automated message is triggered during these hours, its delivery will be delayed until quiet hours are over.

Quiet Hours Period

The default Quiet Hours are 9pm to 10am. This means that if an automated message is triggered to send within these hours, it will be delayed until 10am the next day. Quiet Hours are based on the subscriber's time zone. If there’s no known time zone, the company time zone will be used.
By default, the period we recommend for quiet hours is between 9 PM - 10 AM. You can also change it based on your audience's preferences. 

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