I can't sleep in the evening. Seems like my biological clock has changed bcos of shift job. Or could this be sth serious?

Sleep changes. Many conditions can affect sleep patterns. If you are over 35 and have no other medical problems, maybe you could try melatonin. Melatonin is the bodies own, natural sleep hormone and is available over the counter. I tell my patients to try taking six mgs at eight in the evening. I take it myself and think it's great. Hopetully, melatonin will help, if not, you may need to get checked out.