Muscle relaxers make you sleepy because they work inside your brain, not directly on your muscles. These medications slow down nerve signaling in your central nervous system to reduce muscle tension and spasms, and that same brain-slowing effect produces drowsiness as a side effect. In clinical trials, roughly 38% of people taking one of the most common muscle relaxers reported drowsiness.
They Target Your Brain, Not Your Muscles
The name “muscle relaxer” is a bit misleading. Most prescribed muscle relaxants are classified as centrally acting, meaning they work in your brain and spinal cord rather than on the muscle tissue itself. They interfere with the nerve signals that tell your muscles to tighten or spasm. By dialing down that neural activity, they reduce muscle tone and pain.
The problem is that your brain doesn’t have a separate switch for muscle-related signals versus everything else. When these drugs dampen nerve activity to ease muscle spasms, they also dampen the broader alertness and arousal systems that keep you awake and focused. That’s why sleepiness isn’t a rare or unexpected side effect. It’s baked into how these medications work.
How Different Muscle Relaxers Cause Drowsiness
Not all muscle relaxers trigger sleepiness through the same pathway. The specific brain chemistry involved depends on which drug you’re taking.
Cyclobenzaprine
Cyclobenzaprine (often sold as Flexeril) is one of the most widely prescribed muscle relaxers, and also one of the most sedating. For decades, researchers assumed its drowsiness came from blocking a brain chemical called acetylcholine. A 2024 study in The Journal of Pharmacology and Experimental Therapeutics found something different: cyclobenzaprine is a potent blocker of histamine H1 receptors, the same receptors targeted by drowsy allergy medications like diphenhydramine (Benadryl). That histamine-blocking action is likely the main reason it makes you so sleepy. The drug also interferes with serotonin and norepinephrine signaling in the brainstem, which contributes to both its muscle-relaxing and sedating effects.
Cyclobenzaprine takes about an hour to kick in, and its effects last 12 to 24 hours. That long duration is why many people still feel groggy the morning after taking it.
Tizanidine
Tizanidine (Zanaflex) works differently. It activates receptors in the brain called alpha-2 adrenergic receptors, which reduce the excitatory signals sent to motor neurons in your spinal cord. Think of it as turning down the volume on the “go” signals your brain sends to your muscles. But those same alpha-2 receptors also play a role in regulating wakefulness, so activating them produces sedation as a direct consequence. Tizanidine’s prescribing information specifically warns that sedation “may interfere with everyday activity.”
Baclofen
Baclofen mimics a natural brain chemical called GABA, your nervous system’s primary braking mechanism. It activates GABA-B receptors, which are found throughout the brain and spinal cord. When these receptors are switched on, they reduce the release of excitatory neurotransmitters and make nerve cells less likely to fire. This calms overactive muscle reflexes, but it also calms brain activity more broadly, producing sedation.
Some Are More Sedating Than Others
The degree of sleepiness varies significantly between different muscle relaxers. Cyclobenzaprine and tizanidine sit at the more sedating end of the spectrum. In clinical trial data, cyclobenzaprine caused drowsiness in 38% of patients, along with dry mouth in 24%.
On the other end, metaxalone and methocarbamol are generally considered less sedating options. The American Academy of Family Physicians notes that metaxalone causes “less dizziness and drowsiness than other skeletal muscle relaxants,” and that both metaxalone and methocarbamol may be useful for people who can’t tolerate the sedation from cyclobenzaprine or tizanidine. The trade-off is that the evidence for their effectiveness is more limited.
Carisoprodol (Soma) kicks in faster than most, within about 30 minutes, but wears off in four to six hours. It can also cause significant sedation, and it carries additional concerns because your body converts it into a substance similar to older barbiturate sedatives.
Alcohol and Other Drugs Amplify the Effect
Because muscle relaxers work by depressing your central nervous system, combining them with anything else that does the same thing multiplies the sedation. Alcohol is the most common culprit. Both substances slow brain activity, and together they can compound each other’s effects on drowsiness, coordination, breathing rate, and heart rate.
Other medications that interact this way include opioid painkillers, benzodiazepines (like Xanax or Klonopin), and tricyclic antidepressants. Combining these with a muscle relaxer can lead to extreme fatigue, dangerously slowed breathing, severely impaired coordination, confusion, and low blood pressure. This isn’t a mild concern. These combinations can become life-threatening.
Driving and Daily Activities
The FDA lists muscle relaxants among the medications that can make driving dangerous. Side effects that affect driving ability include sleepiness, blurred vision, dizziness, slowed movement, and difficulty paying attention. Some muscle relaxers carry specific warnings to avoid driving or operating heavy machinery for several hours after taking a dose.
With cyclobenzaprine lasting up to 24 hours, this isn’t just about the first couple of hours after you swallow a pill. Many people find that the drowsiness lingers well into the next day, especially during the first week of use. If you’re starting a new muscle relaxer, it’s worth testing how it affects you during a time when you don’t need to drive or do anything requiring sharp focus.
Managing the Drowsiness
The most straightforward strategy is timing. Taking your dose at bedtime lets you sleep through the peak sedation. For shorter-acting options like carisoprodol, the drowsiness window is narrower (four to six hours), making evening dosing more practical. For longer-acting drugs like cyclobenzaprine, morning grogginess may still be an issue.
If daytime drowsiness is a serious problem, a less-sedating muscle relaxer like metaxalone may be worth discussing with whoever prescribed your medication. Adjusting the dose can also help, since sedation tends to be dose-dependent. Some people find the drowsiness lessens after the first week as their body adjusts, though this doesn’t happen for everyone.