I've found it best, from a weight loss point of view to cycle (or run) in the morning before breakfast unless it's over 25 (cycling) miles or so. That way my body has to start burning fat as it has little choice. As far as improving other aspects of fitness though I'm sure others will be able to advise.
I ride at 5:00 am 4 mornings during the week for around 90 mins to 2 hours then Sat/Sun I go out at 6 am and do 6 or 7 hours total for the 2 days. This allows me to work and also spend time with my family.
I've heard that it's always good to have a coffee before you go out. Apart form the obvious caffeine kick that you get apparently it stimulates your fat cells meaning the bad fat gets pushed into working first, thus helping you lose fat quicker.
Green tea is also supposed to be good for raising the metabolism. Not sure if the 3 or 4 cups a day I drink have helped me shift any of the 5 stone I've lost since February but I certainly don't think it's hindered me.
Caffiene makes the body release sugar into the blood and stimulates the adrenal glands to produce adrenalin, hence why you feel great after a cup of strong coffee. Rapid sugar level rise isn't really that good though, as it will drop just as quickly afterwards and leave you feeling crap unless you eat something to stabilise the sugar levels. Also over stimulation of adrenals can be a bad thing if you are under any other stress - the endocrine sytem is already stressed and over-worked, so adding any more stress (in the form of stimulants) will exaccerbate existing conditions and increase your risk of developing viral infections. I saw some research that suggested that around 4pm was the best time to excercise - that's when the majority of sporting world records are set, apparently. You'll have to Google for it though, sorry! Edit: It seems it may be sports specific - though the first article matches what I said - between 4 and 5pm is best for most sports: http://www.pponline.co.uk/encyc/0616.htm http://www.pponline.co.uk/encyc/0081.htm http://www.pponline.co.uk/encyc/chronobiology-and-the-internal-clock-35845
