Can B12 Make You Feel Sleepy All the Time? Separating Fact from Fiction

January 11, 2026 •

4 min read

An estimated 3.6% of Americans are deficient in vitamin B12, a condition often leading to significant fatigue. Yet, some people report feeling inexplicably sleepy after taking B12 supplements, leading to confusion about its effects.

Quick Summary

B12 deficiency commonly causes fatigue and weakness by impairing red blood cell production. While B12 is more likely to boost energy, high doses may impact sleep cycles in some. Other health or lifestyle factors are also common culprits behind chronic tiredness.

Key Points

Deficiency vs. Excess: B12 deficiency is a well-known cause of fatigue and tiredness, not usually excessive B12 intake.
Red Blood Cells: A lack of B12 impairs red blood cell formation, preventing adequate oxygen transport and causing exhaustion.
Melatonin Regulation: Some evidence suggests high doses of B12 might affect melatonin synthesis and the circadian rhythm, potentially leading to disrupted sleep patterns in some people.
Conflicting Research: The link between high-dose B12 and sleepiness is not conclusive, with studies presenting conflicting findings on its impact on sleep quality and duration.
Consider Other Causes: Many other factors, such as iron or vitamin D deficiency, sleep disorders, or stress, are more likely to be the true cause of chronic sleepiness.
Consult a Doctor: If you experience constant sleepiness, it is essential to consult a healthcare professional to identify the underlying cause and determine the appropriate course of action.

The B12 Paradox: Deficiency Causes Fatigue

At its core, vitamin B12 is known for its role in supporting energy levels, not hindering them. B12 is essential for several vital bodily functions, including nerve function, DNA synthesis, and most importantly, red blood cell formation. A deficiency in this nutrient can lead to megaloblastic anemia, where the body produces abnormally large, dysfunctional red blood cells. These cells cannot effectively carry oxygen throughout the body, causing the profound feelings of fatigue and weakness that are classic symptoms of B12 deficiency. For individuals with low B12 levels, supplementation is designed to reverse this process and restore energy.

Symptoms of a B12 Deficiency

Beyond fatigue, low B12 can manifest in a variety of other physical, neurological, and psychological symptoms. These can develop slowly over time, making them easy to overlook or misinterpret as signs of aging or other conditions.

Neurological issues: Numbness, tingling sensations (pins and needles) in the hands and feet, or difficulty with balance and walking.
Cognitive difficulties: Memory problems, confusion, and 'brain fog'.
Psychological symptoms: Mood changes, irritability, anxiety, and depression.
Physical signs: A sore, red, or inflamed tongue (glossitis), pale skin, weight loss, and a rapid heartbeat.

Investigating High-Dose B12 and Reported Sleepiness

Given that B12 is known for boosting energy, why do some people experience sleepiness after taking supplements? The answer lies in the complex and sometimes contradictory relationship between B12, the circadian rhythm, and sleep-regulating hormones like melatonin. While a B12 deficiency can disrupt sleep, some research suggests that high doses of B12, particularly certain forms, can alter the body's sleep-wake cycle. Vitamin B12 plays a part in melatonin synthesis, and excessive amounts could potentially interfere with this delicate process, leading to disrupted circadian rhythms. However, it's important to note that studies on this effect are conflicting, and not everyone experiences it. Some people may also have paradoxical reactions to B vitamins due to genetic factors or other underlying health conditions.

Other Potential Culprits for Excessive Tiredness

If B12 supplementation isn't the primary cause of your constant sleepiness, a variety of other factors could be to blame. It's crucial to consider these possibilities and consult a healthcare professional to identify the root cause.

Insufficient or poor quality sleep: Not getting enough hours or having disrupted sleep due to issues like light and noise.
Underlying sleep disorders: Conditions such as obstructive sleep apnea (OSA), insomnia, or restless legs syndrome are common causes of daytime fatigue.
Lifestyle factors: A poor diet lacking essential nutrients like iron and vitamin D, chronic stress, or a sedentary lifestyle can all contribute to fatigue.
Other medical conditions: Issues like anemia (unrelated to B12), diabetes, or thyroid problems can cause persistent tiredness.
Medications: Certain prescription and over-the-counter drugs can have side effects that include drowsiness.

B12 Intake: Deficiency vs. Supplementation


Feature	B12 Deficiency	B12 Supplementation (High Dose)
Energy Impact	Causes fatigue and weakness due to impaired red blood cell production.	Typically boosts energy, but some may report temporary sleepiness or changes to the sleep cycle.
Sleep Cycle	Often leads to insomnia or restless sleep due to the body's struggles with regulation.	Some studies suggest it can affect circadian rhythms or melatonin regulation, potentially causing daytime drowsiness or insomnia.
Primary Goal	To correct an underlying nutritional shortage and restore normal bodily function.	May be taken to proactively boost energy or for other perceived benefits, sometimes leading to an excess.
Other Symptoms	Pins and needles, cognitive issues, mood changes, and weight loss.	Can cause mild side effects like headache, nausea, or diarrhea. In rare cases with injections, extreme tiredness.

What to Do If B12 Makes You Tired

If you believe B12 supplements are making you feel constantly sleepy, there are several steps you can take. First, it is crucial to consult a doctor. They can perform a blood test to check your B12 levels and rule out a deficiency or other potential causes of your fatigue, such as iron deficiency or thyroid issues. If you are supplementing under medical supervision, your doctor may suggest adjusting the dosage or the timing of your intake. Some individuals find that taking B vitamins earlier in the day helps minimize any potential impact on sleep. In general, focusing on a balanced diet rich in whole foods is the most sustainable way to get necessary nutrients and support overall energy levels.

Conclusion

While the idea that B12 could make you feel sleepy all the time is confusing, it is important to distinguish between deficiency and supplementation. A B12 deficiency is a much more common cause of profound fatigue and tiredness. The reported sleepiness from high-dose supplementation is a less common phenomenon, likely stemming from complex interactions with the body's sleep-wake cycle in certain individuals. It is not a typical side effect for most. The most effective way to address persistent sleepiness is to consult a healthcare professional to get an accurate diagnosis and rule out common culprits like nutrient deficiencies, sleep disorders, or lifestyle factors. Addressing the root cause is the key to restoring your energy and overall well-being.

For comprehensive information on vitamin B12, visit the National Institutes of Health's Office of Dietary Supplements fact sheet.

Frequently Asked Questions

While it's not a typical side effect, high doses of vitamin B12 may cause temporary sleepiness or fatigue in some individuals, possibly by affecting the body's sleep-wake cycle. However, this is not a widespread or well-established effect.

Yes, vitamin B12 is known to play a role in the synthesis and regulation of melatonin, the hormone that controls the sleep-wake cycle. Disruptions to this process, either from low or potentially very high B12 levels, can impact sleep.

The most common causes include insufficient or poor-quality sleep, undiagnosed sleep disorders (like apnea or insomnia), other nutritional deficiencies (such as iron or vitamin D), chronic stress, or underlying medical conditions.

Yes, fatigue is one of the primary symptoms of vitamin B12 deficiency. A lack of B12 impairs the body's ability to produce healthy red blood cells, which leads to megaloblastic anemia and feelings of weakness and tiredness.

The only way to know for sure is to consult a healthcare professional. They can order a blood test to check your B12 levels and help rule out other potential causes of fatigue, such as sleep apnea, iron deficiency, or thyroid problems.

Since B12 is water-soluble, excess amounts are typically excreted in urine, and toxicity is rare. However, high doses can cause mild side effects like headaches, nausea, or diarrhea. In some cases, it may affect the sleep cycle.

To minimize any potential impact on your sleep, especially with higher doses, it is often recommended to take B12 supplements earlier in the day. However, individual reactions vary.