Valerian root (Valeriana officinalis and V. dioica, both native to the temperate parts of Europe and Asia) is widely available as an herbal supplement; while it has no FDA-approved medicinal use, it is touted as a sleep aid and anticramping agent.
Medical studies have shown that it acts as a mild sedative/hypnotic and has anti-anxiety properties. Thus, it's often recommended to people suffering from mild insomnia or restless legs syndrome, because it's not nearly as "druggy", dangerous, or expensive as over-the-counter or prescription sleeping pills.
The typical dosage is 300-600 mg of the root extract in capsule form taken shortly before you go to bed. A useful dosage is hard to pin down because the quality, purity, and strength of the herb may vary widely between manufacturers. The active chemicals in valerian root seem to be valepotriates, sesquiterpenes and valeric acid. However, no long-term medical studies on the safety of the herb have been done in the U.S.
In my opionion, valerian smells dreadful. If you take it regularly, you will be imbued with its assy stink. Some people have found that valerian upsets their stomach, gives them diarrhea, or makes them groggy.
Valerian also acts as a dream enhancer for many people. While this may be a cool thing for many folks, it's not so good if the reason you've been having insomnia is because you keep having nightmares. Chances are good that while valerian will help you get to sleep initially, it will make your nightmares much worse and you won't get much rest.
Some people also seem to develop a tolerance for valerian and it stops working for them after a while.
If valerian doesn't work for you as a sleep aid, try taking 250-300 mg of magnesium along with 300-600 mg of calcium citrate with a full glass of water an hour before you go to bed. I've had good luck with this combination so far. If you have heart rhythm issues, though, check with your doctor first because the magnesium might cause you problems. Don't be surprised if the magnesium gives you a bit of diarrhea, but the calcium should buffer you against the effect.
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