Because your eyes get dry, causing irritation and redness.
Human eyes are usually covered with a tear film – a “liquid layer which provides oxygen and nutrition”, according to Dr Dana Blumberg.
“When we sleep and the eyes are closed, the lids provide a watertight seal that prevents the tear film from evaporating and allows the eyes to recuperate,” she says.
Lack of sleep prevents this, causing inflammation of the surface of the eye – and “small, superficial vessels dilate” and make the eye look bloodshot, she adds.
The bloodshot vessels show up on the white part of the eye, called the sclera.
Tiredness is just of many possible causes of bloodshot eyes, including allergies and irritation caused by dust or other particles.
Human tears are a “complex mixture of water, fatty oils and mucus” and any disruption to normal production can cause irritation.
Increased evaporation – caused by anything from a windy day to not blinking enough while staring at a computer screen – can also make your eyes bloodshot.
In some cases, bloodshot eyes can be caused by more serious problems – see the NHS guide here.