I suppose everybody likes the time when the night comes and they can go to sleep with the delightful feeling of sleepiness. The problem occurs when such feeling also appears during the day, e.g. at school, at work or when you have to go for training! Where does such feeling come from?
First reason: insufficient amount of sleep at night
There's no doubt that the most frequent cause of the feeling of constant somnolence is sleep defficiency. Nowadays sleep is perceived as “waste of time” and many people simply resign from it. For one or two days you cannot harm yourself when not sleeping enough, but in longer perspective it is disastrous. Besides, the body starts to stand up for itself and generate the feeling of sleepiness at different times than night. Obviously, people differ with the demand for sleep, and even the needs of one body may be various depending on many factors, such as illness, mental stress or physical effort. Nevertheless, the rule of necessity of providing proper amount of sleep concerns everybody.
Second reason: poor-quality sleep
As it was mentioned above, the reason of sleepiness is usually... lack of sleep. However, it may be caused by insufficient amount of sleep or its poor quality. In order for the night recovery process to undergo with proper intensity, the following conditions are necessary:
no exposure to light,
not so high temperature,
It's important not to go to sleep hungry or too full. In the first case it will be hard to fall asleep, and in the second – it's easy to fall asleep, but the sleep is of worse quality (you can wake up sleepy despite 8-hour sleep). It's best to eat the last meal 2 -3 hours before going to sleep. Sitting in front of the computer before going to sleep, or falling asleep with TV on is also adverse. You have to take care of the proper conditions, otherwise you will experience sleepiness during the day.
Third reason: bad diet
Inappropriate way of nutrition may cause the feeling of somnolence – it's a fact that may be surprising for some. The reason of this dependency is the combination of various mechanisms. Leaving the complicated matter of neurology and biochemistry, it's easy to observe that after consuming some kinds of meals, there occurs the feeling of relaxation, or even sleepiness. Such effect can turn up if a meal fulfills the following conditions:
it is bountiful / rich in calories,
it contains fat and carbohysrates – usually highly glycemic,
it is hard to digest,
it contains the addition of sweeteners.
Sometimes even one of those conditions is enough for the feeling of sleepiness to occur. When planning a diet, it's good to balance meals so that not to make them too big (you can divide them into smaller portions) or not to mix refined carbohydrates with fat.
Some people may suffer from sleepiness because of paricular kinds of food, e.g. dairy products and some cereals. The mechanisms of this phenomenon are not clear yet, but supposedly opioid peptides are responsible for that. However, it's only speculation.
Nutrition insufficiency may be of some improtance, too. Although there is not enough information concerning specific dependencies, there are reasons to believe that selective malnutrition may influence the mood and foster the feeling of tiredness.
Fourth reason: too many stimulants
In case of constant use of stimulants, such as caffeine, there may occur a specific reaction of the body, which is sleepiness after particular dose of that compound. Many people can notice such effect. Additionally, it's good to know that overusing stimulants – even if you don't feel the booster – may contribute to worsening of the quality of night sleep. Therefore, it's important not to use stimulants in the evening.
Fifth reason: health disorder
The feeling of sleepiness may be also a symptom of a disease. The typical example is... narcolepsy – its symptom is excesive sleepiness and the incidents of sudden sleep. That's extreme, obviously, as this disease is very rare and sleepiness during the day has different reasons far more often, and it may also be a disease. Such feeling accompanies people suffering from e.g.:
Sixth reason: overwork
The feeling of sleepiness is sometimes a natural consequence of the overwork of the body, which is caused by too much work, education, training... or stress connected with other reason than those mentioned above. In case of sportsmen and very active people, overtraining, which is the result of disproportion between the intensity of training stimuli and the adaptation possibilities of the body and may get visible in various ways, is highly possible. One of the symptoms of overtraining is, in fact, the feeling of constant fatigue with apathy and somnolence.