There are two fiction books that give you a bit of an insight into the thought process of being an F1 - "Trust Me I'm a Junior Doctor" by Max Pembleton and "Bodies" by Jed Mercurio. Both authors were/are actually doctors and are very loosely semi-autobiographical but heavily juiced up in places. You have to be a bit careful with books though as they tend to be written before the EWTD when F1s worked 65+ hour weeks in a live-in arrangement at the hospital. By the time you get to 3/4/5/6th year you'll have friends from medical school who are F1s/F2s etc. so you will get a good idea of what the lifestyle is like.
From what I've seen, my imagine is of a fairly busy and reasonably stressed job especially at the start of F1 - but generally fun. Once they find their feet they seem to be a bit better. Out of work a lot of hospitals have quite a tight social group of juniors and they seem to be fairly happy. Its a grind job but the novelty of having some pocket money for the first time in 5/6 years makes life a bit easier - nicer houses, better food, not constantly in panic about money, more money to socialise and holidays.
Some juniors make out that it's the worst time of their life. Maybe it is - I don't know. I suspect its more likely either their specific circumstances or their personality (i.e. the usual medic types catastrophising about everything). On the whole it seems like a pretty good lifestyle, and in many respects an easier lifestyle than medical school in the sense you've gotten past finals 'fear of failure', have a reasonable degree of job security, finances and time to sort yourself out. To be honest it sort of has to be like that - because you couldn't operate the way you do at medical school forever. Obviously work stress is a bit different to academic stress, and some medics are quite inexperienced in workplace enviornments (e.g. dealing with petty work-politics) which is probably a bit of a shock to the system, as even as a clinical student you tend to avoid that sort of atmosphere and exist in your own little academic learning bubble.