Yesterday, i can't sleep. i'm always coughing before i go to bed. i had drink a glass of hot lemon, but, i still coughing. what should i do?

Nighttime cough: It could be a few different things, but most commonly cough at night (when you lie down) is either from post nasal drip or reflux (acid from the stomach coming up) irritating the airways, or from cough variant asthma, which after bother people more at night. It's usually a process of elimination treating potential problem and seeing if there's improvement. Warm drinks can help, honey helps too.

Possible reflux: Sometimes people that have gastroesophageal reflux have acid come up the esophagus when they lie down. This can cause coughing. I would try taking a proton pump inhibitor such as omeprazole about an hour before bed. Other things you could try are not eating within 3 hours of going to bed. Avoiding alcohol and caffeine and raising the head of the bed a few inches. If no help please see doctor.