Apple has issued a stark warning to smartphone addicts who sleep while holding their devices during charging.

In the online user guide, the tech giant states that "iPhones" should only be charged in a "well-ventilated area" - that is, on a flat surface rather than on a thick blanket.

iPhones heat up during charging, so if this heat has no space to escape, it can potentially cause burns or even start fires.

This is why keeping your charging phone under your pillow is one of the worst things smartphone users can do.

Apple fails to tell people not to charge their phone overnight at all - although this is the official advice from some fire services.

The tech giant states: "Prolonged contact with warm surfaces can cause discomfort or injury." It explains with the following narrative:

Use common sense to avoid situations where your skin is in contact with a device, power adapter, or wireless charger when it is running or plugged into a power source for long periods. For example, do not sleep on a device, a power adapter, or a wireless charger, or place them under a blanket, pillow, or your body when connected to a power source.

- Keep your "iPhone," power adapter, and any wireless charger in a well-ventilated area during use or charging.

- Exercise particular caution if you have a physical condition that affects your ability to detect heat.

Apple does not explicitly advise against charging your phone overnight at all; in fact, as long as it is in a "well-ventilated area," it should be safe, as suggested by Apple's blog.

However, some fire services go a step further by advising against any type of overnight charging.

Another reason not to charge your iPhone overnight relates to how it affects the lifespan of your device.

Phones typically take up to two hours to fully charge, but when plugged in overnight, they could be on for eight hours or more, depending on how long you sleep.

This increases the amount of heat the battery is exposed to and ultimately reduces its lifespan, meaning you’re likely to have to purchase a new phone sooner.