Insomnia is when you cannot sleep, or cannot stay asleep, night after night — not just one late evening of worry. Some people cannot fall asleep; some wake at three and lie there; some sleep in broken pieces. Doctors treat it as a real problem, not a small complaint. Though actually, people use the word a bit loosely: after one bad night they say “I have insomnia,” when they only slept badly. True insomnia lasts. It can come from stress, from screens, from tea and coffee late, from a loud street, or from the body itself. The word is ordinary enough that you will hear it in daily talk, not only at the clinic. People also just say “I can’t sleep.” Formal twin: sleeplessness. The person who lives with it is an insomniac; the trouble itself is insomnia.