We all lack energy from time to time but if it doesn't improve then you should see your GP.
There are a number of conditions that can leave you feeling lethargic. Iron levels are one of the first things that can affect energy levels and cause tiredness.
An under-active thyroid is another cause of tiredness and the falling hormone levels that occur at the menopause.
Fatigue can also be a sign of diabetes.
If you are suffering from SAD (Seasonal Effective Disorder) this can also cause fatigue.
As well as the above, some medications can also cause lethargy, including beta blockers, some antihistamines, codeine-based painkillers and also some antidepressants. Also some sleeping tablets may help to get you through the night, some can cause daytime fatigue.
Of course, anxiety, stress and depression are also triggers for sapping energy levels. The best course of action is to go and visit your GP.