Feeling tired all the time is called fatigue, which is when tiredness is overwhelming and is not relieved with rest. Tiredness is able to be alleviated with rest.
There are numerous causes for fatigue and low energy, including:
- Iron deficiency anemia, which can be caused by
- Heavy menstrual periods
- Ulcers
- Taking nonsteroidal anti-inflammatory drugs (NSAIDs)
- Vitamin B12 or folate deficiency anemia
- Iron-overload disorder (haemochromatosis) (rare)
- Chronic fatigue syndrome
- Sleep problems
- Sleep apnea, a condition in which people stop breathing for short periods during sleep
- Restless legs syndrome
- Lack of sleep
- Other sleep disorders
- Underactive thyroid (hypothyroidism)
- Celiac disease, an autoimmune condition in which the immune system responds abnormally to a protein called gluten
- Diabetes
- Depression
- Anxiety
- Infectious mononucleosis (“mono”)
- Rheumatoid arthritis
- Cancer
- Fatigue may be from cancer itself or from the treatments such as chemotherapy and radiation
- Certain medications, such as antidepressants, antihistamines, pain relievers, and medicines for nausea
- Recovering from major surgery
- Infections
- Heart disease
- Kidney disease
- Liver disease
- Chronic obstructive pulmonary disease (COPD)
- Untreated pain
- Fibromyalgia
- Emotional stresses, including grief, financial or personal stresses
- Boredom
- Lifestyle habits
If you've been tired with low energy for several weeks and sleep does not alleviate the symptoms, talk to your doctor. Treatment for fatigue and low energy depends on the cause.
From 
Healthy Resources
References
https://www.nhs.uk/live-well/sleep-and-tiredness/10-medical-reasons-for-feeling-tired/
https://www.nia.nih.gov/health/fatigue-older-adults
https://www.nia.nih.gov/health/fatigue-older-adults