When I’m feeling tired and / or demotivated and have work to do I usually try to push myself harder, largely out of concern that if I don’t push myself I will feel tired more often. However, I don’t feel that this is really working for me.
Other ways of dealing with this might be to;
What do you do, if you begin to feel tired and demotivated?
fall asleep… 
All of those, but I also put on some good music or I put on a documentary or another tv programme I might enjoy. Keeping your brain stimulated really helps. (I also listen to Radio 4 online)
I often push myself to finish just a section and then have a proper break for a hot drink, a snack and maybe a half-hour of tv.
As I have no contact with the outside world except via the internet, any tv watching is strictly voluntary - I know exactly how long a programme will last, I can choose to spend half an hour watching a cooking show or something if I like, with my coffee and a sandwich!
On the other hand for marathon work sessions I really miss having a proper television that can be left on in a corner 24/7, with no need to keep choosing “good” things to watch. Really good tv is wasted when you are concentrating on working! But it provides something to rest your eyes and brain on every once in a while, keeping you from getting too tired.
It seems somehow that actually a big part of the “tiredness” is your brain getting fed up with looking at the same things and thinking about the same things. A little bit of variety helps a lot.
Lately it’s been music a lot of the time. Good for creative thinking.
Ideally I would have all three, and someone to bring me drinks and food at sensible intervals!
Oh, and smoking a joint really helps. Am I allowed to say that here? The longer the session the more painful it can get, and these things help stamina. If you have been awake and working for almost a solid week with a few naps, the only effect something like that has is to make it hurt less.
I am really finding that being in an isolated environment without the usual types of stimulation (tv, friends, smokes) is seriously affecting my productivity.
Now I will expect to be slated. But I am absolutely serious. We are all different and I’m just answering your question honestly.
I have to say that since I quite coffee four months ago, I have been generally feeling a lot less tired, even though I sleep about 5-7 hours a night.
I listen to some jazz music it really calms me down and relaxes me.
Music with no lyrics and a good beat or tune. Techno or Classical, whatever. Most likely music with lyrics will distract you with singing along.
Take a walk or take a nap. You either need to get the body moving or the body resting.
I usually take a little break, and if I’m still not up to it I put on some fun, peppy music to distract myself. Either the music gets tuned out when I’m focusing on creative work or I go on autopilot for regular coding and focus on the music. 
:sing:
If I feel burnt out then I take a break from the computer screen and go read a book or a magazine for a few minutes.
I will almost always take a break and do something totally different (i.e. no screens). I find that if I’m too tired my work suffers so much that it’s not even worth ‘pushing through’ it. Better to take a break, refresh, and try again.
And then there’s copious amounts of coffee 
Thanks for the helpful suggestions everyone, I will definitely start listening to music more to give me an energy boost.
Ya music can rejunevate you and you would start feeling enthusiastic very soon.
Taking a break with some fresh air is wonderful and certainly invigorates you. Then getting back and putting your fave music one will get you right in the mood for doign soem great work!
I look at my stack of bills. That gets me motivated!
Feeling tired: shine a bright light in my face. It works. The brain responds to light.
Demotivated: take a break, go for a walk, or something. Nothing sucks more than burnout. When you are burnt, you are burnt.