Artificial lights set your internal clock to "awake" when you should be asleep, suppressing the release of the sleep hormone melatonin. On the contrary, turning off the lights prior to bed gives your body the signal that it is night-time and sleep-time.  Use light-restricting black out curtains. Switch to lower-wattage lamps as you get ready for bed. Use a night light for late-night toilet runs. Put away digital clocks with glowing displays. Make sure the room is at a comfortable temperature for you. The human body temperature should be cooler that during daytime hours - most experts suggest the temp should be at least 5 to 10 degrees cooler. It depends on your personal preference, but the best sleep is between 60 and 67 degree Fahrenheit. You may be able to drift off to sleep with background noise, but, in the wee hours of the night, sound can easily pull you from your slumber. If you fall asleep to music, set a timer so that it goes off 20 to 30 minutes later. If your bedroom is positioned near a noisy intersection, or your neighbors are rambunctious night owls, grab a pair of earplugs to ensure a peaceful night's rest. If you are talking on the phone, watching TV, or eating late-night snacks in your bed, your body will find it hard to associate this zone for resting. Make the bedroom a technology-free zone. Use your bed primarily for sleeping and sex, so that falling asleep gets easier.

TL;DR in Vietnamese:
Giữ cho phòng được tối. Giữ phòng ở nhiệt độ thích hợp. Hạn chế tiếng ồn. Dùng phòng ngủ cho việc ngủ một cách nghiêm túc.