Your Circadian rhythm is your body's natural sleep-wake cycle, also known as your "body clock".
As the sun sets, your brain tells your body to release melatonin, which makes you sleepy and ready for bed. When the sun rises your body stops releasing melatonin and releases Cortisol to boost energy and wake you up.
As you can imagine, nowadays a lot of the time our bodies have no idea when it is daytime and when it is night-time. We are constantly exposed to unnatural light at night which decreases our melatonin.
Most of us don’t get enough sunlight during the day, especially in the morning, negatively affecting cortisol production. (Too much cortisol is bad, and too little is also bad).
At night, the blue light from your cellphone or TV, causes your brain to decrease melatonin production. The blue light causes your body to think that it's daytime, and it releases cortisol instead of melatonin, making it much harder to fall asleep. Even if you do fall asleep, your sleep quality is much poorer.