Apparently, it's artificial light, especially the one emitted by various electronic devices, such as TVs, tablets and computers.
Artificial light after dark suppresses the body's production of melatonin, a very important hormone since it regulates, among other things, blood pressure and glucose levels. Melatonin shortage can increase the risk of cancer, diminish your fertility, disrupt your sleeping patterns, affect your immune system and put you at risk for obesity and heart disease.
Laptops and other electronic gadgets are especially dangerous since the blue wavelengths they use are the most disruptive at night. Staring at computer screen after dark is basically equivalent to staring at the bright sun and it's not something you should be doing at night.
There are special programs which adjust the colours of your monitor to the time of day. One of them is called f.lux. Also read this article on the danger of tablets.
It's interesting that while many people are obsessed about their diets few appear to realise that there is danger lurking in something as seemingly innocent as electric lights! If you want to know more on the topic, just google melatonin and artificial light.