Barbiturates are drugs that act as central nervous system depressants, and by virtue of this they produce a wide spectrum of effects, from mild sedation to anesthesia. Barbiturates are derivatives of barbituric acid. They are also effective as anxiolytics, hypnotics and as anticonvulsants. They have addiction potential, both physical and psychological. Barbiturates have now largely been replaced by benzodiazepines mainly because benzodiazepines are significantly less dangerous in overdose. Ultrashort-acting barbiturates are commonly used for anesthesia because their extremely short duration of action allows for greater control. The middle two classes of barbiturates are often combined under the title "short/intermediate-acting." These barbiturates are also employed for anesthetic purposes, and are also sometimes prescribed for anxiety or insomnia. The final class of barbiturates are known as long-acting barbiturates (most notably phenobarbital, which has a half-life of roughly 92 hours). This class of barbiturates is used almost exclusively as anticonvulsants, although on rare occasions they are prescribed for daytime sedation. Barbiturates in this class are not used for insomnia, because owed to their extremely long half-life, patients would awake with a residual "hang-over" effect and feel groggy.