Why does one loss all the energy after half a day?

sleep disorders, stress, insufficient sleep poor eating habits can also cause excessive sleepiness at this time.

Eating habits: your own eating habits may make you fatigued :

            It takes a lot of intestinal blood to break down the food after eating and deliver it to the liver, which puts more pressure on the blood.  As a result, the blood pressure decreases and the body feels tired and falls asleep৷ .

Don’t eat  :      Much heavy meat or fatty foods

Eat :                    Keep a light meal like a lot of salads, vegetables and soups on your lunch list, egg,  a bowl of cereal with skim milk and a glass of juice or piece of fresh fruit
   Two slices of toast with peanut butter and a banana

            If your lunch tends to be carb-heavy, this can contribute to your tiredness. Eating carbohydrate loaded foods like white rice, white bread, pasta or chips can make you feel sleepy because they cause a rapid spike in blood pressure, which is inevitably followed by a dramatic drop soon after.
   

            This will reduce the pressure on the blood and intestines, which means that the food will be easily digested  As a result, you will not feel ‘tired‘ .

If you’re feeling sluggish in the early afternoon, ask yourself these questions:

                     1;  Did I eat breakfast?
                     2: What did I eat for breakfast? (A cup of coffee doesn’t count.)
                      3: What did I eat for lunch? Was it from the vending machine?
                _Your answers may point to the problem.

Body system:

        It has to do with a dip in your core body temperature .Right before you go to sleep at night, your core temperature begins to drop, which is a signal to the brain to release melatonin _(a hormone associated with relaxation and sleep.). The exact same thing happens on a smaller scale between 2 and 4 in the afternoon. It’s a mini-signal to your brain to get sleepy.”  

     To combat this, make the effort to boost your Serotonin levels during this period. Serotonin is essentially the opposite of melatonin; a hormone linked to increased energy and focus ;

                1: Listen to upbeat music,
                2: get a few minutes of sunshine or go for a brisk walk to boost your energy levels.
         

Dehydration:

      Fluids are vital for our energy levels. In fact, even mild dehydration can leave you feeling tired, sluggish and moody — and this can start to happen after losing just 1.5% of your body’s water weight according to the National Sleep Foundation.