Blue light from technology screens like smart phones is known to interfere with sleep. Suppression of melatonin, a naturally occurring hormone, is the reason. It helps us fall asleep and makes sleep restorative. Researchers recently demonstrated that blue light also interferes with your body's ability to naturally reduce its temperature at night, which also enhances restorative sleep. During the night, this temperature drop hits its low at about 4 a.m. As body temperature returns to normal, you begin to awaken. Blue light inhibits this temperature drop. This in turn causes your deep sleep period to suffer. Hence, you are tired the next day.