GP Dr Sindhu Siddiqi of the Smart Clinics, London, explains: "Most of the time, fatigue is because of long working hours, poor sleep, emotional stress and dehydration.
"But sometimes there are more serious, underlying problems which warrant further investigations."
Here, she outlines some symptoms that are best to get checked.
1. You've been tired for a long time
You're getting enough rest, but you're tired and constantly thirsty. This may suggest you have diabetes. Other symptoms are weight loss, constipation and recurrent infections.
2. You're exercising and eating healthily but you can't lose weight and you're tired
You're cold when others aren't, you're constipated, and if you're female, your periods aren't regular. You might have an underactive thyroid. The thyroid gland produces a hormone called thyroxine, which is essential for the smooth running of nearly all our organs.
3. You're tired doing activities you could manage easily before
You're feeling more breathless and your heart's beating faster. You might be anaemic, which means your red blood cells aren't transporting enough oxygen around your body, so it's working harder to compensate. Anaemia can be due to not getting enough iron and vitamins in your diet (iron and vitamin B12 deficiency can happen easily in vegans). It can be because your body isn't absorbing them properly, as in coeliac disease.
4. You're tired and losing weight without trying
Any unintentional weight loss with a normal diet suggests your body is using more calories than it should. Tiredness associated with weight loss can sometimes be due to cancer where the uncontrolled division of cells leads to more energy being used. Any lumps in your neck, armpits or groin that persist for over three weeks require further investigation to rule out lymphoma, a blood cancer that can happen at any age.
5. You're tired and uncoordinated
Tiredness is an early symptom in 80% of people affected by multiple sclerosis (MS). If you have other symptoms too, such as electric shock sensations, balance or coordination problems, have a chat with your doctor.
6. You have joint pains and you're tired
This can be a sign of rheumatoid arthritis, which is different to the wear-and-tear arthritis osteoarthritis. Rheumatoid arthritis is an autoimmune condition where your body triggers inflammation in your joints for an unknown reason.
If you're worried about any symptoms, visit your family doctor.