Have you ever felt tired around 2 pm, just after lunchtime? It doesn't mean you're lazy! Around this time, your body has a big dip in energy, same as 3-4 am (that's why it's not that difficult to fall back to sleep around this time).
Did you know that you've got a pretty accurate internal clock? This "clock" is your brain's way to make sure your body functions properly all throughout the day, yes, a 24 hour — or very approximate — day.
Right on the base of your brain, in the hypothalamus, there's a unique structure called the suprachiasmatic nuclei, commonly referred to as "The Master Clock". This structure is made up of approximately 50,000 individual "clock cells" which take part in different biological processes such as protein production.
But this internal clock wouldn't work without it being exposed to the external world, and this is where LIGHT has an important role!
Depending on the degree of light (or darkness) your eyes are exposed to, your brain will send different signals to your body that will vary through the day. This is called circadian rhythms, from the Latin, circa dies, which means 'about a day'.
When it's dark at night, your eyes send signals to your hypothalamus that you should feel tired, and the brain does the rest, also there are times you'll feel with more energy! Every hour of the day will have a different impact on your internal clock, and in consequence, in your body.
This is why proper lighting in your home and workplace is so important! Don't mess around with your clock! Having that white fluorescent lightbulb over your head all day long won't help you at all. If you want to be more productive, raise your energy levels, and sleep better, implementing circadian lighting (or lighting that varies in intensity and color throughout the day to simulate natural light) at your place can be the answer.
If you have the chance, go get some work done outdoors, the lighting is perfectly set-up there!
Listen to your body, it knows what it needs!