I just can't work an 8 hour day
That is fine - especially if I read it as: "I cannot work non-stop 8 hours a day every day with maximum efficiency". That is not a good target, especially considering the negative impact on your health.
It is actually very recommended to take breaks from work, in order to avoid certain work-related health problems. The recommendations depend on the type of work.
For IT people, usually the rule is 10 minutes of break (away from any technology) after every 50 min of work. Of course, this cannot be implemented literally, but gives a good clue about the realities of life.
Please remember that even robots need downtime and maintenance. The very heart in your body (everybody's body, for that matter) rests after every beat.
I spend an hour stuck on something and then get it done in 5 minutes the next morning.
That is normal. When the brain is tired (possibly because of overload), it cannot work any more. A rested brain however, will find no issues in dealing with complex problems.
This thinking is at the base of the decisions taken by many companies to provide their employees with resting places. Even 5-10 min of passive rest (laying horizontally, maybe even napping) brings huge benefits compared to the employee being a zombie in front of the monitor. I know that from personal experience.
Is this a normal thing to experience?
It depends on many factors, but it is not unheard of. It differs from country to country, from company to company, from person to person.
Is there anything I can do about it?
Well, yes. Some ideas:
- Take occasional breaks. Search the internet for details on what to do during those breaks, based on your workplace's specifics.
- Try to spend a bigger part of your free time in green areas (parks, forests). Oxygen helps the brain work better (as well as it helps the entire body).
- Increase the intake of fresh vegetables and fruits, especially the ones strongly colored. Consult a dietitian / nutritionist, if you are not sure how to do that. A deficit of vitamins and minerals can "help" you getting tired sooner.
- Consult a doctor, ask for a "review" of your health, potentially finding the cause of the fatigue. Follow their recommendations.
- Consult an ophthalmologist, you might need (new) glasses. This is a great factor, easily not understood, especially by people who did not wear glasses before.
The manager has noticed this and regularly reminds me to try to stay focused in the afternoon and put in more effort. The problem is I am already putting in my full effort. I'm not playing on my phone or reading reddit, I just can't get anything done in the last hour of the day.
Disclaimer: this is an occasional workaround. Do not use the following statements as a solution, it can (and probably will) hurt your job on the long term.
If one time you are tired and you cannot have a break, but you know that you can catch up easily later or during the next day, or if you are ahead with your work anyway, then pretend to work for one hour. Do not just look on the ceiling, do not use smartphone, don't chat with your colleagues, to keep them from doing their work. That will avoid you getting on bad terms with your boss.
Reminder: this is not a solution, do not do it often, to avoid penalties in the future.