Yes, MK-677 (ibutamoren) commonly causes tiredness, especially during the first few weeks of use. Fatigue and lethargy are among the most frequently reported side effects, alongside increased appetite and water retention. The U.S. Food and Drug Administration has specifically listed fatigue as a known side effect of ibutamoren. The good news for current users: most people find the tiredness fades as their body adjusts, though it can persist in some cases depending on dose and timing.
Why MK-677 Makes You Sleepy
MK-677 works by mimicking ghrelin, the hormone your body releases when you’re hungry. Ghrelin doesn’t just trigger appetite. It also plays a role in sleep regulation, stress hormone release, and energy balance in the brain. When you take MK-677, you’re essentially flooding your system with a synthetic ghrelin signal, which activates pathways that naturally wind the body down.
That ghrelin-like activity triggers a surge in growth hormone, which peaks during deep sleep under normal conditions. By amplifying growth hormone output, MK-677 pushes your body toward the same physiological state it enters during rest. A study published in the Journal of Neuroendocrinology found that MK-677 increased deep sleep (stage IV) duration by roughly 50% and REM sleep by more than 20% in young subjects compared to placebo. In older adults, REM sleep increased by nearly 50%. Sleep deviations dropped from 42% under placebo to just 8% on higher doses. In other words, MK-677 genuinely makes your sleep deeper and longer, which can spill over into daytime drowsiness, particularly if you’re not used to that level of sleep intensity.
Blood Sugar Swings Add to the Fatigue
Growth hormone and insulin have an antagonistic relationship. When growth hormone goes up, your cells become less responsive to insulin, a condition called insulin resistance. MK-677 reliably causes this effect, and it’s one of the most well-documented side effects. The DEA lists insulin resistance as one of the most common consequences of use, and long-term use may increase the risk of developing type 2 diabetes.
What does insulin resistance feel like day to day? Your cells struggle to pull glucose from your bloodstream efficiently, so even though there’s plenty of fuel circulating, your muscles and brain can’t access it as readily. The result is energy crashes, brain fog, and that heavy, sluggish feeling after meals. If you’re eating large carbohydrate-rich meals (which is easy to do, since MK-677 dramatically increases appetite), the blood sugar rollercoaster gets worse. You spike, your body over-releases insulin to compensate, and then you crash. That cycle alone can explain persistent daytime tiredness that doesn’t improve with more sleep.
Water Retention Plays a Role
MK-677 causes noticeable water retention in many users. The FDA flagged this as a significant side effect, noting it can even increase the risk of congestive heart failure in certain individuals. While most users won’t face anything that serious, carrying extra water weight contributes to a feeling of physical heaviness and sluggishness that many people describe as fatigue. Your body is working harder to move more fluid, your joints may feel stiffer, and the overall sensation is one of being weighed down. This isn’t the same as true neurological fatigue, but it stacks on top of the sleep and blood sugar effects to make tiredness feel worse than any single cause would suggest.
Timing Your Dose to Reduce Tiredness
MK-677 has a half-life of about 24 hours, meaning one dose per day keeps blood levels relatively stable regardless of when you take it. That said, timing can shift when the drowsiest window hits. Many users take MK-677 before bed specifically to work with the fatigue rather than against it. A nighttime dose aligns the drowsiness with your natural sleep window and amplifies growth hormone release during the hours your body already produces the most. The tradeoff is that the appetite spike can make it hard to fall asleep on an empty stomach.
A morning dose spreads the sedative effect across waking hours, which can make it milder but more persistent. Some users prefer this because the hunger side effect is easier to manage during the day. If daytime lethargy is your main concern, a bedtime dose is generally the better starting point. One practical note: taking MK-677 right before a heavy carbohydrate meal blunts the growth hormone response, since insulin suppresses it. Spacing the dose away from large meals may help reduce both the blood sugar crashes and the fatigue that comes with them.
How Long the Tiredness Lasts
For most users, the worst fatigue hits during the first one to two weeks. Your body is adjusting to consistently elevated growth hormone and IGF-1 levels, and the ghrelin receptor activation is a new stimulus your central nervous system hasn’t adapted to. Most people report that lethargy gradually improves as tolerance develops. This doesn’t mean the sleep-enhancing effects disappear. The deeper sleep tends to persist, but the carryover grogginess during waking hours typically becomes less noticeable.
If fatigue doesn’t improve after several weeks, insulin resistance is the more likely culprit than the direct sedative effect. Chronically impaired blood sugar regulation doesn’t resolve with tolerance the way drowsiness does. It tends to worsen with continued use. Monitoring how you feel after meals, particularly carb-heavy ones, can help you distinguish between the two. Post-meal energy crashes that get worse over time point to a metabolic issue rather than simple adjustment-phase sleepiness.
The Paradox: Better Sleep but More Fatigue
One of the more confusing aspects of MK-677 is that it objectively improves sleep quality while simultaneously making people feel more tired during the day. The sleep study data is genuinely impressive: fewer disruptions, more deep sleep, more REM sleep. For older adults in particular, MK-677 appeared to restore a more youthful sleep pattern. Yet users still report daytime lethargy as a top complaint.
This makes more sense when you separate the causes. The sleep improvement is a direct effect of growth hormone on sleep architecture. The daytime fatigue comes from a combination of insulin resistance, water retention, and the sedative properties of ghrelin receptor activation that don’t shut off just because you woke up. You’re sleeping better at night and still feeling tired during the day because the tiredness isn’t primarily a sleep-quality problem. It’s a metabolic and hormonal one.