That's an interesting question, and there is no clear-cut answer. Morning goes from midnight to midday. So 3 am is 'three in the morning' - but as it's dark, and most people are sleeping, you could also call it 'night'. Afternoon technically starts at midday, although in a working or school environment we would say 'morning' for the period before the lunch break (usually around 12.30 -1.30) and 'afternoon' for the period after the lunch break. Evening falls (begins) when it starts to get dark - bearing in mind that the period of dusk is longer in the UK than it is in Spain. Obviously, evening falls at different times depending on the time of year (in Scotland it can be as early at 3 pm or as late as 11pm), but on average it's 6pm in most of the UK. So if you want a general guide as to when afternoon stops and evening begins, you could say it's around 6 pm. Night is when it is dark and most people are sleeping.