The iHealth Wave tracker records all your activity (physical activity, exercise, swimming and sleep) from 0h to 24h. It is reset every day at midnight (number of steps, distance traveled, calories burned, level of activity ...). The data not synchronized before midnight are obviously not lost, they are stored in memory until the next synchronization.
For a fast synchronization, daily synchronization is recommended. But the iHealth Wave can record up to 14 days of data.
If you have not synchronized your iHealth Wave within the 14 days, the oldest data in memory will be overwritten and replaced with the new ones.