(Dec 16) WHEN it comes to energy levels in the body, it is not simply food in, energy out.

Understanding your body's natural bio-rhythms can make you fitter, happier, more productive.

At any time, our brain takes up a quarter of our body's energy. So, if we are already running on empty, our thinking becomes muddled and it feels like we have to work harder to achieve the same output. To work smarter, we need to match high-energy tasks with high-energy periods.

Our heart rate, metabolic rate, breathing rate and body temperature are regulated by circadian rhythms.

This natural cycle is just over 24 hours and is linked to the amount of sunlight and darkness we are exposed to. However, our circadian rhythms can be artificially regulated by alarm clocks or stimulants like caffeine.

There are also shorter biological cycles, called ultradian rhythms, of about two hours in duration.

These changes in hormonal and brainwave activity affect our sleepiness, alertness or hunger.

During the first part of the cycle, we are full of energy and drive, but after about 90 minutes or so, our body starts to signal that it is going into a recovery phase before the next cycle. So during the next 20 minutes, we feel our concentration wane, we get hungry, feel tired and start making mistakes.

If we follow this rhythm, office meetings should not last for more than 90 minutes, or people will start fidgeting and stop listening.

We can structure our time so that we can become more efficient during the 'peaks' and chill out during our 'lows', instead of trying to plough through a project for several hours.

Our current lifestyles are not based on our body's natural rhythms but have come about after the Industrial Revolution. Working eight to 10 hours with just a lunch break in between was designed to get the most work out of the average factory worker.

IN THE MORNING

Immediately after waking, our energy levels are low but we are also extremely calm and relaxed.

At the first signs of light (before we are actually awake), our bodies start producing adrenalin and noradrenalin to stimulate the brain to wake up.

The levels of 'sleep hormone' melatonin, which peak before going to bed, are near their lowest at this time as well.

This is the best time to be doing creative work, when the body is awake but we still have access to our dreaming state. This is also a good time to do meditation in preparation for the day ahead.

However, early morning is not the best time to schedule a work-out at the gym.

In a British study, subjects were asked to perform the same work-out at four different times - 5am, 11am, 5pm and 11pm. They found the 5am slot physically the hardest.

This is because our body temperature is at its lowest at 4am (and also at 2pm). It slowly rises during the day, warming up our muscles and improving our physical performance.

However, working out in the early morning seems to give a better emotional and mental boost than evening work-outs. The solution?

Keep your morning work-outs gentle and save the hard stuff for the evenings and weekends.

AFTER LUNCH

Researchers into sleep deprivation have found that there is an increased tendency to sleepiness and reduced capacity to function between 2 and 5pm, whether or not the subject has slept.

Most of us know from practical experience that this is the worst time to hold a meeting or discuss important matters.

It is better to use this time to do routine or 'auto-pilot' type work and schedule crucial tasks during high-energy periods.

Having low energy also means you can be cranky and more prone to arguments.

In another study, subjects were asked to rate conditions that annoyed them, and the same condition scored higher in the afternoons than in the mornings.

By late afternoon, our energy levels start to rise again. Body temperature, muscle strength, reaction time and hand-eye coordination improve throughout the day and peak between 4pm and 7pm, making this the best time to exercise.

Not surprisingly, most Olympic and world records are won in the late afternoon.

BEFORE BED

Any exercise or stimulating activity after 7pm could be counter-productive as your body is beginning to wind down for the night.

The endocrine and immune systems increase their activity just before bedtime, while the natural level of cortisol, the hormone released when we are stressed, is at its lowest.

Traditionally, people used to get up before sunrise and go to bed after sunset.

This may be our body's natural rhythm after all. If you notice, as people age, they tend to revert to early mornings and early nights. Staying up till the wee hours of the morning seems to be the privilege of the young.

To stay up late, your body needs to produce more adrenalin and this can lead to an unhealthy cycle of restless sleep and bleary mornings.

If you really want to understand your body's bio-rhythms, the best way is to keep a diary for a week and record your energy levels and moods.