Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a stimulant drug. Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants. It is most commonly consumed by humans in infusions extracted from the seed of the coffee plant and the leaves of the tea bush, as well as from various foods and drinks containing products derived from the kola nut. Other sources include yerba mate, guarana berries, guayusa, and the yaupon holly. In humans, caffeine acts as a central nervous system stimulant, temporarily warding off drowsiness and restoring alertness. It is the world's most widely consumed psychoactive drug, but, unlike many other psychoactive substances, it is both legal and unregulated in nearly all parts of the world. Beverages containing caffeine, such as coffee, tea, soft drinks, and energy drinks, enjoy great popularity; in North America, 90% of adults consume caffeine daily.[5] Caffeine is toxic at sufficiently high doses. Ordinary consumption can have low health risks, even when carried on for years Ц there may be a modest protective effect against some diseases, including certain types of cancer. Caffeine can have both positive and negative effects on anxiety disorders.[citation needed] Some people experience sleep disruption if they consume caffeine, especially during the evening hours, but others show little disturbance and the effect of caffeine on sleep is highly variable. Evidence of a risk to pregnancy is equivocal, but some authorities have concluded that prudent advice is for pregnant women to limit consumption to the equivalent of two cups of coffee per day or less.[6] Caffeine has diuretic properties when administered to people who are not used to it, but regular users develop a tolerance to this effect, and studies have generally failed to support the common notion that ordinary consumption contributes significantly to dehydration. With heavy use, strong tolerance develops rapidly and caffeine can produce clinically significant physical and mental dependence. Coffea arabica (pron.: /r?b?k?/) is a species of Coffea originally indigenous to the mountains of the southwestern highlands of Ethiopia. It is also known as the "coffee shrub of Arabia", "mountain coffee" or "arabica coffee". Coffea arabica is believed to be the first species of coffee to be cultivated, being grown in southwest Arabia for well over 1,000 years. It is said to produce better coffee than the other major commercially grown coffee species, Coffea canephora (robusta), but tastes vary. C. arabica contains less caffeine than any other commercially cultivated species of coffee. Wild plants grow to between 9 and 12 m (29 and 39 ft) tall, and have an open branching system; the leaves are opposite, simple elliptic-ovate to oblong, 6Ц12 cm (2.4Ц4.8 in) long and 4Ц8 cm (1.6Ц3.2 in) broad, glossy dark green. The flowers are white, 10Ц15 mm in diameter and grow in axillary clusters. The fruit is a drupe (though commonly called a "cherry"; interestingly, the plural form is simply "cherry" - used only when referring to the fruit of C. arabica - when referring to the actual cherry fruit, the appropriate plural is "cherries") 10Ц15 mm in diameter, maturing bright red to purple and typically contains two seeds (the coffee seeds).