The "initial estimated time to sync" shown on the sleep progress card will get adjusted to a more accurate number in few minutes when sufficient data is synced.

The amount of data generated (and saved on the device) depends upon the various options you have chosen.

Biometric scanning (RED and Infra-Red LEDs) is done at 43 Hz and lasts for 45 seconds. The accelerometer runs at 10Hz.

e.g., If you are doing biometric reading every 5 minutes (12 times each hour) and sleep for 8 hours, the amount of data generated would be

For Infra-Red Signal = 43 * 45 * 12 * 8 = 185,760
For Red LED Signal = 43 * 45 * 12 * 8 = 185,760
For Accelerometer = 10*60*8*60 = 288,000
Total Data points = 659,520

We need all that data to compute various biometric measurements accurately.

This is why we suggest opening the app multiple times a day, or at least making sure background syncing is activated so that the app can sync the data while running in the background.