The principal metric would be mood, however defined. Zeo’s web interface & data export includes a field for Day Feel, which is a rating 1-5 of general mood & quality of day. I can record a similar metric at the end of each day. 1-5 might be a little crude even with a year of data, so a more sophisticated measure might be in order. The first mood study is paywalled so I’m not sure what they used, but Shiotsuki 2008 used State-Trait of Anxiety Inventory (STAI) and Profiles of Mood States Test (POMS). The full POMS sounds too long to use daily, but the Brief POMS might work. In the original 1987 paper A brief POMS measure of distress for cancer patients, patients answering this questionnaire had a mean total mean of 10.43 (standard deviation 8.87). Is this the best way to measure mood? I’ve asked Seth Roberts; he suggested using a 0-100 scale, but personally, there’s no way I can assess my mood on 0-100. My mood is sufficiently stable (to me) that 0-5 is asking a bit much, even.
Spinach is rich in the antioxidant lutein, which is thought to help protect against cognitive decline, according to researchers from Tufts University. And a longitudinal study at Harvard Medical School found that women who reported eating the most leafy green and cruciferous vegetables had a markedly lower rate of cognitive decline, compared to those who ate the least.

But while some studies have found short-term benefits, Doraiswamy says there is no evidence that what are commonly known as smart drugs — of any type — improve thinking or productivity over the long run. “There’s a sizable demand, but the hype around efficacy far exceeds available evidence,” notes Doraiswamy, adding that, for healthy young people such as Silicon Valley go-getters, “it’s a zero-sum game. That’s because when you up one circuit in the brain, you’re probably impairing another system.”
Recent findings also suggest that taking extra vitamins could help preserve memory, especially as we age. Researchers at Australia's University of Sydney tested 117 people in a retirement home by putting them through a battery of mental tests that included remembering a string of words, listing as many words as possible that begin with a certain letter of the alphabet, and doing mental addition and subtraction. Those who regularly took vitamin C, they found, scored higher on the tests.
It’s a frosty Monday evening in March, but in the back of Idea Coffee, a dingy café near the Empire State Building, things are heating up. A group huddles around a small black box—the $160 ApeX Type A brain stimulator, with its retro-looking meter and dial and two electrodes. It’s supposed to bolster learning by delivering a mild electric current to the brain. The guy who’s been experimenting with it for a week notes that the only thing he’s noticed so far is a metallic taste in his mouth.
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The use of prescription stimulants is especially prevalent among students.[9] Surveys suggest that 0.7–4.5% of German students have used cognitive enhancers in their lifetime.[10][11][12] Stimulants such as dimethylamylamine and methylphenidate are used on college campuses and by younger groups.[13] Based upon studies of self-reported illicit stimulant use, 5–35% of college students use diverted ADHD stimulants, which are primarily intended for performance enhancement rather than as recreational drugs.[14][15][16] Several factors positively and negatively influence an individual's willingness to use a drug for the purpose of enhancing cognitive performance. Among them are personal characteristics, drug characteristics, and characteristics of the social context.[10][11][17][18]