Does Lysine Make You Tired and Sleepy? The Surprising Truth

October 24, 2025 •

4 min read

Interestingly, research suggests that a deficiency in lysine, an essential amino acid, is more likely to cause fatigue and tiredness rather than the supplement itself. This directly contradicts the common myth and prompts a closer look at how lysine impacts the body's energy production and sleep cycle.

Quick Summary

Many wonder if taking L-lysine causes fatigue, but evidence points to the opposite, with a lack of this amino acid potentially causing tiredness. We explore the roles lysine plays in energy production and its relationship with sleep and anxiety.

Key Points

Lysine and Fatigue: A deficiency in lysine, not the supplement itself, is a more common cause of fatigue and poor concentration.
Role in Energy: Lysine helps produce carnitine, a nutrient crucial for cellular energy production from fatty acids.
No Sedative Effects: Lysine is not a sedative and does not typically induce sleepiness in the average person.
Individual Reactions: For a small number of people, lysine can have a stimulating effect, especially when taken before bed.
Minimal Side Effects: Side effects are primarily gastrointestinal and occur with high dosages, not drowsiness.
Consult a Professional: Anyone experiencing persistent fatigue should seek medical advice to identify the root cause, rather than assuming it's from a lysine supplement.

Understanding the Myth: Does Lysine Cause Fatigue?

Contrary to popular belief, lysine is not known to cause tiredness or sleepiness. In fact, an inadequate intake of this essential amino acid can be a contributing factor to feeling constantly tired and lacking concentration. The misconception likely stems from a misunderstanding of how the body uses lysine and its role in overall metabolic function. While it's not a stimulant like caffeine, its function in the body is more aligned with maintaining energy levels and supporting overall health, rather than inducing lethargy.

The Role of Lysine in Energy Production

Lysine plays a critical role in several bodily functions, including the production of energy. As an essential amino acid, the body cannot produce it, so it must be obtained from dietary sources or supplements. One of its key functions is synthesizing carnitine, a nutrient that helps transport fatty acids into the mitochondria of cells where they are burned for energy. This process is vital for sustaining energy levels and preventing fatigue. Without sufficient lysine, carnitine levels can drop, leading to decreased energy and a general feeling of weariness.

Lysine Deficiency vs. Lysine Supplementation

It is crucial to distinguish between the effects of a lysine deficiency and those of supplementing with lysine. A shortage of this amino acid can lead to a cascade of negative effects beyond fatigue, including:

Poor concentration
Loss of appetite and unintentional weight loss
Nausea and dizziness
Delayed recovery after illness

Conversely, supplementing with lysine is intended to correct a deficiency or support specific health goals, such as managing herpes outbreaks or improving calcium absorption. When taken as recommended, supplementation should alleviate deficiency-related symptoms like fatigue, not cause them. For most individuals, taking lysine supports a healthy metabolic rate and improved energy synthesis, leading to greater overall vitality rather than sleepiness.

Why Some People Report Sleep-Related Effects

While most evidence indicates lysine is not a sedative, there are anecdotal reports of sleep-related effects. For a small number of people, particularly those with pre-existing anxiety, disturbed sleep patterns, or hyperactive thinking, taking lysine before bed can be stimulating rather than calming. It is important to note that this is an individual reaction and not a widespread effect. Other potential reasons for anecdotal reports of sleepiness could include:

Correcting Deficiency: If a person's fatigue is caused by a lysine deficiency, taking a supplement can restore balance, leading to a state of normal restfulness that is mistaken for sedation. For someone constantly fighting weariness, a return to feeling 'normal' might be perceived as sleepy.
Psychological Factors: As with many supplements, the perception of its effects can be influenced by expectation. If a person anticipates a calming effect, they may perceive one, even if it is not a direct pharmacological action of the amino acid.

Potential Side Effects and Safe Dosage

Generally, lysine is considered safe for most people when consumed in appropriate doses. The most commonly reported side effects are gastrointestinal, such as stomach pain, nausea, and diarrhea, particularly with excessive intake. Dizziness and headaches have also been reported in some cases. To minimize side effects, it is best to stick to the recommended dosage and consult a healthcare professional, especially if you have an existing medical condition or are taking other medications. For more detailed information on amino acid functions, a good resource is the Cleveland Clinic on Amino Acids.

Lysine Effects: Deficiency vs. Supplementation


Feature	Lysine Deficiency	Lysine Supplementation	High-Dose Side Effects
Energy Levels	Causes persistent fatigue and low energy	Supports energy production and vitality	Not typically impacted, but high doses may cause issues
Sleepiness	Not directly related, but fatigue can cause daytime sleepiness	Does not cause drowsiness for most; can be stimulating for some	Not a direct side effect
Concentration	Leads to poor concentration	May improve focus by alleviating deficiency symptoms	Not a reported side effect
Stomach Issues	Not a direct symptom	Generally well-tolerated	Can cause stomach pain, diarrhea, and nausea

Conclusion: Unmasking the Real Culprit of Fatigue

In summary, the notion that lysine makes you tired or sleepy is largely a myth. The reality is quite the opposite: a lack of lysine is far more likely to cause fatigue, poor concentration, and other symptoms of low energy. As an essential amino acid, lysine is integral to energy metabolism by facilitating the production of carnitine. While a very small number of individuals, particularly those with anxiety, may experience a stimulating effect from nighttime doses, this is an individual reaction, not a typical side effect. The most common side effects from high doses are related to gastrointestinal discomfort, not drowsiness. If you are experiencing persistent fatigue, it is always recommended to consult with a healthcare professional to determine the underlying cause, as it is unlikely to be your lysine supplement.

Frequently Asked Questions

For some individuals with anxiety or disturbed sleep, taking L-lysine at night can be stimulating rather than calming, potentially leading to restless sleep or insomnia. Most people do not experience this effect.

The most common side effects associated with L-lysine, particularly at higher doses, include gastrointestinal discomfort such as stomach pain, nausea, and diarrhea.

Lysine may help some individuals manage anxiety by reducing the stress response. However, for a small number of people, particularly those already prone to anxiety, taking it at night can be stimulating. It's best to monitor your personal reaction.

A deficiency in lysine can significantly impact energy levels by limiting the body's production of carnitine, which is vital for burning fatty acids for energy. This can cause persistent fatigue and tiredness.

For most people, taking lysine in the morning or at a time that works best for them is fine. If you are sensitive or have anxiety, it might be better to avoid taking it close to bedtime to prevent potential stimulating effects.

Feeling tired is not a typical adverse reaction to lysine. In fact, it is more likely a symptom of a lysine deficiency rather than supplementation. Other side effects are usually gastrointestinal.

There isn't enough reliable information to know how lysine interacts with sleep medications. It is crucial to speak with a healthcare provider before combining supplements and medications, especially for sleep.