Advice...um. Eat three meals per day with some protein with each one. I like this list of foods, too. They're recommended for mood, but they are good for general brain health.

You're already getting enough sleep. That's good. If you have a device that runs apps (smartphone, tablet) try an app like SleepBot to monitor your sleep for 3 nights or so, just to see if anything unusual turns up. Do you normally feel rested and good when you wake up in the mornings?

When you take a break, are you able to take a short walk? If not, doing something indoors like stretching or some breathing exercises can help you to feel more alert. Singing is good, too. Basically anything to get you to take really deep breaths and get some fresh air into your lungs.

I don't think I've heard of 5-HTP making anyone tired. That doesn't seem like a problem.

I don't know if you do any kind of regular exercise or yoga. I can't claim to be doing any myself lately, but I know when I worked at a desk for long hours, the exercise seemed to help me to keep my energy level up. It also helped to keep my anxiety level down.

I hope you figure it out. Some medical condition like anemia or hypothyroidism could be going on. I'd suggest going to the doctor to ask them, but I see you're in the UK, so I don't know if they'd do a blood test on the first visit. If you're female, an iron supplement can be good (a low-dose, slow-release one is best.)