5 mins read

What Makes Us Tired? Top 9 Causes Of Low Energy

August 27, 2021

General wellbeing

Woman sat on bed looking tired

Blog Homepage >>

Feeling tired all the time can almost be accepted as part and parcel of everyday life. From a busy, stressful job, to taking care of a family there is a lot in today’s modern lifestyle that can drain our energy reserves.

So, what makes us low on energy? We take a look at the top 9 reasons why we might be feeling low on energy and tired all the time.

9 Reasons We Feel Tired

1. Poor Sleep

Top of the list is a poor night’s sleep. Adults should be aiming to get around 8 hours sleep a night, but according to YouGov’s sleep tracker, just 18% of adults achieve this with 31% getting 7 hours sleep a night and 28% getting 6 hours sleep a night.

The causes of poor sleep range from stress, anxiety, or depression to hormone imbalance, as well as lifestyle factors such as smoking, lack of exercise and a poor diet high in caffeine, alcohol, and sugary foods.

Learn more about why sleep is so important to our health.

Woman sat up in bed

2. Lack of Vitamin D

Vitamin D is well known for bone health, but did you know that low vitamin D levels can also be a cause of fatigue?

In a study we carried out into vitamin D deficiency we found that 74% of respondents had below optimum levels and 27% had insufficient or deficient levels of vitamin D.

UK adults should be getting 400UI or 10 mcg of vitamin D a day.

Learn more about Vitamin D.

Infographic of vitamin d statistics in the uk

3. Iron Deficiency Anaemia

Iron deficiency anaemia is caused when there is not enough iron store in the body. This not only causes fatigue and low energy levels but pale skin, headaches, increased heartbeat and dry skin and hair.

Ferritin is the main storage protein for iron and is crucial for keeping the amount of iron in the body balanced. Ferritin is, therefore, a good measure of iron levels and low levels can indicate iron depletion which can indicate iron deficiency anaemia. An underactive thyroid (more on that later), and vitamin C deficiency can also lead to iron-deficiency anaemia. Women who have heavy periods are susceptible to low iron levels.  

4. Low Magnesium Levels

Magnesium is a mineral found in the body that plays a key role in functions such as energy production. Being deficient in magnesium can cause fatigue as well as a loss of appetite, nausea, vomiting, weakness, muscles contractions and cramps.

5. Lack Of Vitamin B12

Vitamin B12 is part of the B complex of vitamins and is gained through the food we eat. Vitamin B12 is needed for the formation of red blood cells which carry oxygen around the body. If we don’t consume enough B12 through our diet, then this can lead to B12 deficiency the symptoms of which include fatigue, lethargy, and feelings of weakness. As the body can store B12, deficiencies can occur over a prolonged period of time. This is also why testing for total B12 can give an inaccurate B12 status. Active B12 on the other hand measures the total amount of B12 available for your body to use rather than stored B12.

Vitamin b12 deficiency symptoms.

6. Underactive Thyroid

An underactive thyroid is when the thyroid gland is not producing enough thyroid hormone. An underactive thyroid can leave you feeling fatigued and low on energy as well as experiencing unexplained weight gain and having an increased sensitivity to cold.

7. Type 2 Diabetes

Type 2 diabetes is a condition where the insulin your pancreas makes doesn’t work properly or your pancreas doesn’t make enough of it. This causes your blood sugar levels to rise and keep rising. With type 2 diabetes your body is unable to get enough glucose into cells, as a result, someone with type 2 diabetes will feel very tired. Alongside this, they will also feel very thirsty, will experience unexplained weight loss and need to go to the toilet a lot more than normal. An HbA1c test will assess your risk of developing type 2 diabetes.

8. Perimenopause

The transition to menopause known as perimenopause is when women experience changes to their natural hormone fluctuations causing a hormonal imbalance.

Declining levels of oestrogen can affect the body’s magnesium levels which help your muscles to relax. Low magnesium levels can therefore impact your sleep. Falling oestrogen levels also cause night sweats which can disrupt your sleep.

Learn more about menopause.

Female hormone mapping

9. Low Testosterone In Men

Testosterone in men is an important sex hormone that regulates libido (sex drive), body mass, strength as well as being involved in the production of red blood cells and sperm. If your testosterone levels are low this can result in fatigue and difficulty sleeping.


Being low on energy can be very frustrating and affect all aspects of your life. It’s important to understand the underlying cause of low energy levels in order to rule out conditions such as thyroid disorders, and nutritional deficiencies. Hormones play a big role in our overall wellbeing, especially in women with the hormone fluctuations that occur across their menstrual cycle. Understanding these fluctuations and how they impact your wellbeing is key to putting the right strategies in place to make you more energised and feel better.

Read Next: ‘8 Tips To Improve Your Energy Levels’>>

Back To Blog >>

This information has been medically written by Dr Thom Phillips

Thom works in NHS general practice and has a decade of experience working in both male and female elite sport. He has a background in exercise physiology and has published research into fatigue biomarkers.

Dr Thom Phillips

Dr Thom Phillips

Head of Clinical Services