there are various causes like thyroid problems anemia vitamin deficiency especially vitamin d and even psychological issues which are the most common. you should get hemoglobin levels thyroid function test and vitamin d levels checked. i suggest you try doing the following: try taking multivitamin supplements like syrup zincovit or a to z two teaspoons once a day for 3 to 4 weeks. replace tea with coffee and have two to three times a day especially before some important work which will provide adequate interest and stimulation. regular exercises or sports will be helpful. gradually taper off addictions if any. you will probably need vitamin d supplements after the test. try doing yoga in the morning. do you have any psychological issues like stress or depressive? if yes then you need to consult a psychiatrist. revert with more information to a general medicine physician online --> <link>
why do i feel weak while sitting and working?