Burnt Sun

Main Idea

Whenever a Entity or Player is out in Daylight (No Blocks above Someone.) They get Damaged (2 Damage (1 Heart)) and Die eventually. (A Person/Entity that is in Water that is in Daylight will still Die)
A Person will still get Damaged at NightTime. (Get blame it on the fact that Moon Light is reflected SunLight)

After a Player Spawns a Oak Leaf Block will be placed above their head so they don’t die instantly.

Sun Sickness

Sun Sickness happens when you stand out in the sun for too long. (300 ticks default (It is a Gamerule))
The Sun Sickness Timer is based on how many ticks a Person has stood in the sun.
You Could see this timer by doing /BurntSun Stats
Sun Sickness does:

• -6 from Max Health.
• -1 from Safe Fall Distance.
• Make a Entity/Player take 2x Damage from the Sun (4 Damage (2 Hearts)).