Why are we so tired in the winter?

It might have to do with the fact that you burn more energy keeping warm and that there is less sun.

Probably seasonal affective disorder. More on that here:
http://en.wikipedia.org/wiki/Seasonal_affective_disorder

During the summer we don't have the furnace turned on and tend to keep the windows open for a fresh breeze to come through our homes. During the winter we have our furnaces turned on, keeping us in warmth and like the little baby who's all bundled up and wrapped in warmth, sleep sets in. Our brain gets tricked into this sleep mode.

I think it's because of lack of sunlight and physical exercise. I proved it last winter. Usually I am tired all winter, but if I exercise consistently during the winter months it gets better.

Less sunlight in winter.

your not more tired in the winter its just that the daylight hours are shorter giving you more time to rest with the sky being dark

Darius Denzel Wesley

Less sun (But when it's out in the Winter, we feel better, but being so far away in Winter, it's not as strong-acting on us as in the Summer.)

No green leaves or grass (It's either white - snow and ice, dead, or muddy. Yuck!)

COLD (wears us out trying to stay warm)

It more boring since we can't "go outside" as much. Even for those of us who DON'T spend a LOT of time outside in the summer, knowing that option is there, and that when we do, we aren't going to 1) have to dress NEAR as warmly, and 2) will NOT be "cold"... So, we tend to stay "trapped" inside where it's warm, watching all those movies we haven't seen yet, reading, playing on the computer, or computer games, and we get bored of less physical activity (which, conversely, keeps us WARM).

Now, all of the above is for those of us in places where it gets cold in the winter and hot in the summer. But even where it stays moderate temps, the sun is further away, and some of the 'best" plants won't grow right. ;-)

I think it is because of the extra amount of energy needed to stay warm and the lack of sunlight and overeating.

I have been a marathon runner for over 20 years and I train outdoors year-round. Such outdoor activities as running and cycling simply require additional investment in layered clothing and specialized fabrics.
Once in the habit, winter is a favorite season. The outdoor activity maintains a steady energy level. The clarity, purity, and cleanliness of the air is invigorating. Seasonal tiredness and resignation is a choice that can be changed.

because more of our energy is used to keep warm.

Seasonal depression.

cuz it gets cold duh...

I find the antithesis to be true! The summer sun ZAPS THE SHIT out of ME!!!!!

Because there's less food in the Winter?

The metabolism of all animals slows down in the winter, some so much that they hibernate. The lack of sun and our distance from the sun has a lot to do with it.

Instead of heeding our bodies and semi-hibernating, or getting the rest our bodies crave, we create all kinds of holidays to give ourselves a focus outside of ourselves. When the artificial holidays are over, we just crash from double exhaustion but at least we are well past the shortest day of the year by then.

melitonion in the body affected by sunlight

you use up more energy in an effort to keep warm..that's why people living in colder climates need to eat more high-energy food than those living in hotter climates :)

Because people eat more in the winter and start to feel lazy while at the same time - it slows down the metabolism with less outdoor sunny activites. The less sunlight means you produce more melatonin which makes you sleepy in the first place. I always do home workouts that include weight training, jump roping and riding my stationary bike for around 30 to 40 minutes. I find it a great way to stay fully motivated, especially in the winter.

It has to do with the amount of natural light we are exposed to. I also feel depressed in the winter time. I bought a type of lamp that provides the exposure so I do feel a little better.

It gets dark very early which pretty much creates an allusion of it being 'bed time' for us. Also we lack exercise, and going outside.

Probably because its so cold and depressing outside, that we mostly stay cooped up indoors and relaxing so we dont have to face the cold weather, so after doing nothing for a few days, you begin to lose energy.

A better answer to this and you can refrence this at http://www.webmd.com your bodys blood when cooler thickens. so the blood moves slower through out your body in which causes fatique faster and more easier. In the summer your blood thins out and moves alot faster. This causes your body to respoind faster and be more alert.