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If you still feel tired after a good night's sleep, you're probably dehydrated. Drink some water after you wake up.

— Just So You Know (@Know) April 22, 2013

Some quick searching unsurprisingly indicates dehydration is a potential source of fatigue (see #10), but is there any medical evidence that dehydration causes fatigue despite proper sleep and that drinking water after waking up will improve wakefulness—as the tweet implies?

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    I recommend clearing up some confusion here - "dehydration results in fatigue" is a separate issue than "drinking a glass of water when you wake up makes you feel alert". I recommend narrowing this down. For instance, the drink might make you more temporarily alert, but still not necessarily hydrated, so the question would remain unanswered (and the test of drinking water upon waking is ambiguous).
    – VCavallo
    Apr 30 '13 at 21:37
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    I'd guess the more likely diagnosis is sleep apnea.
    – Mark Hurd
    May 4 '13 at 2:57
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It could be a sign but it could also mean other things. When it comes to your health and symptoms you see there can be many causes.

https://www.risescience.com/blog/waking-up-tired

One of the many restorative processes happening in your brain as you sleep is the flushing out of adenosine, a chemical that builds up during waking hours and eventually causes feelings of sleepiness and the desire to crawl into bed. During the night, the body clears out the built-up adenosine.

However the chemical residue doesn’t just magically disappear the moment you wake up. It can take anywhere from 60 to 90 minutes for it to dissipate and for its effects to wear off. It's a necessary transition and not an indicator of poor sleep.

The blog also mentions hydration

Hydration: After eight hours of sleep, the body is usually dehydrated, and when you’re parched, it can be difficult to focus and think clearly.

https://www.mayoclinic.org/diseases-conditions/insomnia/symptoms-causes/syc-20355167

Insomnia symptoms may include

Not feeling well-rested after a night's sleep

Stress. Concerns about work, school, health, finances or family can keep your mind active at night, making it difficult to sleep. Stressful life events or trauma — such as the death or illness of a loved one, divorce, or a job loss — also may lead to insomnia.

Travel or work schedule. Your circadian rhythms act as an internal clock, guiding such things as your sleep-wake cycle, metabolism and body temperature. Disrupting your body's circadian rhythms can lead to insomnia. Causes include jet lag from traveling across multiple time zones, working a late or early shift, or frequently changing shifts.

Poor sleep habits. Poor sleep habits include an irregular bedtime schedule, naps, stimulating activities before bed, an uncomfortable sleep environment, and using your bed for work, eating or watching TV. Computers, TVs, video games, smartphones or other screens just before bed can interfere with your sleep cycle.

Eating too much late in the evening. Having a light snack before bedtime is OK, but eating too much may cause you to feel physically uncomfortable while lying down. Many people also experience heartburn, a backflow of acid and food from the stomach into the esophagus after eating, which may keep you awake.

https://www.fda.gov/consumers/consumer-updates/always-tired-you-may-have-sleep-apnea

Do you feel tired or have a headache when you wake up in the morning? Something might be going wrong while you sleep that you don’t know about: obstructive sleep apnea (pronounced app-nee-uh).

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