Your iPhone screen goes dark mainly due to several automatic features and conditions designed to protect the device and save battery life. Common reasons include:

• Auto-Brightness: Your iPhone adjusts screen brightness based on ambient light to save battery and reduce eye strain. This can make the screen dimmer automatically in darker environments.

• Overheating: If the iPhone gets too hot, the screen will dim or go dark to help cool down the device.

• Low Power Mode: When the battery is low, your iPhone may dim the screen to conserve power.

• Attention-Aware Features: If enabled, the screen dims when your iPhone detects you are not looking at it.

• True Tone and Night Shift: These features adjust the color and brightness for a better viewing experience but can sometimes cause the screen to appear darker.

• Software glitches or settings like Dark Mode can also dim the screen.

You can try fixing unwanted dimming by turning off Auto-Brightness in Settings

Accessibility > Display & Text Size, disabling Attention-Aware Features, charging your phone, letting it cool down, or adjusting True Tone and Night Shift.

If your screen goes completely black or won’t turn on, a restart or force restart may be needed.

These automatic dimming features mainly aim to improve battery life, protect the phone hardware, and enhance user comfort by adapting to surrounding conditions.