It's 3 o'clock in the afternoon, and you're looking for something to haul you out of the energy slump. The top choices are often sugar- or fat-laden snacks from a vending machine or beverages loaded with caffeine.

Sweet snacks can actually create surges and dips in blood sugar, which can make you feel more lethargic. And fatty foods do little for your energy levels. That donut may have seemed like a good idea when you were famished, but, soon, it's as though you can feel it sitting in your gut, weighing you down.

Beverages meant to give you an energy jolt can also have the opposite effect. Just one cup of coffee may help keep you energized for up to six hours, but caffeine can trigger a cycle of fatigue by interfering with REM sleep. Alcohol poses a double whammy; it causes the body to lose nutrients, and it warps sleep patterns.

Here's a secret to revitalization, particularly effective for the afternoon blahs: Treat yourself to a tall, ice-cold glass of water.

Water has wonderful restorative properties. It is a natural, fat-free appetite suppressant that contains no calories and no cholesterol. It is low in sodium, helps the body metabolize fat, helps maintain skin and muscle tone, and improves energy levels.

Every physiological function depends on water. Water helps regulate body temperature, transports oxygen, nutrients, hormones, and antibodies; helps eliminate toxins and other wastes from the body; and lubricates your joints as well as your hair, skin, mouth, nose, and eyes.

Water protects organs and tissues; increases the efficiency of proteins and enzymes essential to metabolism; and relieves water retention (though it may seem counterintuitive, when you're retaining water, the best course of action is to drink more water, not less).

If you allow yourself to get dehydrated, every part of your body suffers. Dehydration has been linked to asthma and allergies, constipation and heartburn, hypertension and headaches, poor muscle tone, and inefficiencies in digestion, metabolism, and organ function.

Warning signs of dehydration include mental confusion, pain in the joints, stomach, and back, and low energy.

Keep your energy levels up, particularly after a workout, by making sure you get enough.

-- How much is enough?

Here's a good rule of thumb: drink one-half of an ounce of water for each pound of body weight. So if you weigh 140 pounds, you need 70 ounces per day.

As with most major lifestyle changes, check with your doctor before significantly changing your water intake. Certain medical conditions call for restricted or increased water consumption.

When you first begin to re-hydrate, you may feel like you're spending all of your time in the bathroom. Just wait it out. Within a few weeks, your body will adjust and you will urinate less frequently. Just avoid heavy water intake right before bed to avoid midnight trips to the bathroom, which can interfere with sleep patterns.

Here are some common excuses for not getting enough water:

-- "I'm not thirsty." You may notice that, when you drink more water, you find yourself thirsty, but, when you live on caffeinated soda, you're not thirsty at all.

The truth is, thirst is not a good indicator of water deprivation. A lack of thirst may actually signal dehydration, and "dry mouth" thirst is a sign of extreme dehydration. When your body is deprived of water, it adjusts by disabling the body's thirst sensor. Once you start hydrating yourself, thirst kicks in again.

-- "I don't like water." Here are some tips for downing the day's water:

* Make it readily available. Keep a water bottle in your car and at your desk. Stash another bottle in your bag and carry it with you.

* Liven up ordinary water with a squirt of lime or lemon juice. Sometimes, I add a splash of fruit juice to the water. Try to go easy on the juice, though. Most juices are high in sugar, which adds calories and stimulates the pancreas.

* Purchase an inexpensive water filter, such as the Brita water filter. For about $30, you can enjoy great tasting water all the time. Plus, when you have cold, refreshing water ready in the fridge, you'll be more likely to pour yourself a glass.

* Find an elegant container. Every drink looks more refreshing in an elegant crystal goblet.

* Know thyself. How is water made most appealing to you? Do you prefer an ice-cold glass with fresh lemon slices? Then make sure it's easy to prepare.

* Experiment. Put a glass mug in the freezer before bed or freeze a half-filled water bottle, and fill it with water before you go out for the day. Then enjoy the cold water as the ice thaws.

When you are properly hydrated, you'll experience an energy boost and you may find that you eat less, too.