What does the light blue color indicate in Sleep app on Apple Watch Series 10?

I would like to know what the color in following from 5 am to 6 am indicate. Apple defined respective colors only for Awake, REM, Core and Deep. This light blue color between 5 am to 6 am is not indicative of any of these colors. So what does this mean, is it a mix of Deep and Core sleep?

[Re-Titled By Moderator]