And that could be bad news for your health, since running yourself down could seriously affect your resistance to viruses and other health problems. The question is, what can you do to boost your immune system so you don't run out of steam?
Here's how to keep your resistance strong:
Eat right It goes without saying that a balanced diet helps bolster your immune system against viral attacks. But if you're finding it tricky to fit a regular square meal into your busy schedule, at least have a good breakfast. Why? Because studies show people who skip breakfast are more susceptible to cold and flu bugs (which is a sure sign of a weak immune system).
Take vitamins If your diet isn't all it should be, one short-term solution is to take a quality multivitamin supplement. Many people also believe taking vitamin C will keep their immune system strong, but there's far more evidence for the immune boosting effects of taking zinc, ideally in lozenge form. Herbal medicine fans, on the other hand, swear by echinacea - though scientists are divided over whether or not it has any effect on immunity.
Try meditation Lots of studies claim meditation boosts the immune system, thanks largely to the fact that it helps relieve stress. Some experts also think it boosts the production of melatonin, a hormone that's good for immunity. Finding time to sit still and clear your thoughts, on the hand, may be a bit more of a challenge.
Visualise Alternatively try a technique called visualisation, which may be easier than meditation for those who find emptying their minds utterly impossible. Simply imagine yourself in a peaceful place (tip: use your other senses too - so what does it smell like, what does it sound like and so on - to make the visualisation more effective).
Take hot and cold showers Go on, grit your teeth and add a few bursts of cold water when you take your morning shower. It's believed to stimulate the thymus gland (which, in turn, stimulates the immune system). At least you'll feel wide awake afterwards.
Quit smoking If you haven't already, that is. Experts believe smoking irritates the respiratory system, which lowers your immunity and makes you more susceptible to viruses. Smoking or staying virus-free? It's a no brainer.
Go to bed Have an early night, at least once in a while. Being tired all the time is, after all, thought to be particularly bad for your immune system.
Keep moving Even when you're flat out it pays to stay active, since exercise is a great stress reliever (and that, in turn, helps boost your immune system). But don't do too much, as excessive exercise can weaken your immunity. Twenty minutes of exercise three or four times a week may be all you need to keep your resistance strong.
Do you have any tips on how to resist colds, flu and other nasties, especially when your life is all go? Share them with us.