It's not really about the time, or about whether it's dark or not. You will never get a straight answer this way, because concepts of 'early' and 'late' vary from one person to the next and one culture to the next. And the sunset idea doesn't work either, because in some parts of the world it is dark before 4pm in winter. This isn't even evening, let alone night. The best way to think of the difference is like this: 'In the evening' means before you go to bed. 'I often watch TV in the evening' refers to the whole period between your evening meal and bedtime. If you normally go to bed at midnight, then 11pm can still be classed as part of your 'evening'. 'At night' refers to the hours of darkness when you, or most other people, are sleeping. For example, 'My sister is a nurse, so she often has to work at night', or 'You should turn off your computer at night to save electricity'. Or, for example 'I often watch TV at night' might suggest, for example, that you wake up in the night and can't get back to sleep. So you maybe sit up in bed and watch TV for a while. There are no hard and fast rules on this one, but I hope this gives you a general idea.

The difference between evening and night is actual time of day. Evening is typically the hours between 6:00 pm and 10:00 pm. Night is generally any hours that the sun is not above the horizon. (sunset - sunrise) (so speaking by definition, it can be night and evening at the same time.) 'I often watch TV at night' without context I would think you watch TV around 10:00 pm and later. 'I often watch TV in the evening' without context I would think you watch TV during/after dinner.

"at night" is later than "in the evening"

Well, my friend... Evening means the beginning of the night ( 6, 7 pm). People usually say "good evening " to greeting, when they arrive and say hello. Night is really night, later (9, 10, 11 pm). People usually say "goodnight " to farewell, to go to bed, to say bye.

Where I live in New York, USA, we generally use evening for after 5pm, and night usually after 8pm. However if we're at a fancy party, or if a man is speaking romantically toward a woman he is not yet dating but perhaps is interested in, I've noticed the language to become more formalized where although it is after 8pm, instead of saying, "tonight" or "have a good night," they will say more formally "this evening" or "have a good evening".