I've seen a fair few studies over the while that suggest caffeine increases arousal in the short term and that for some tasks, this will also increase performance (there's a few citations here).However, as with most drugs, habituation leads to withdrawal effects and adjustment, so that in some senses once a person gets used to caffeine, having a regular dose of caffeine may just bring a person to something like a normal level of functioning.Thus, in a general sense, for someone used to consuming a certain level of caffeine, does having that regular quantity increase arousal or performance over and above the level that would be obtained at that moment had the person never got in the habit of consuming caffeine in the first place?
Does caffeine improve performance for habituated consumers?
psychopharmacology;arousal;habituation
Nehligh's 2010 reviewNehlig (2010) wrote a systematic review article called Is caffeine a cognitive enhancer? (link to publisher, no PDF).On page S89 to S90 Nehlig reviews the evidence of caffeine as a cognitive enhancer within the context of a history of caffeine consumption:In  caffeine non-deprived young (2025 years) and older subjects (5065  years), 250 mg caffeine slightly offset the decline in performance  found in the placebo group on a digit span memory test. However,  caffeine did not improve performance in the immediate word recall  task in either age group [11]. Caffeine (12.5100 mg) given after  overnight abstinence improved cognitive performance in a relatively  difficult and stressful task involving rapid visual information  processing and using a high load of working memory in 11 males and 12  females, aged 1856 years. The effect was more marked in individuals  with a high habitual caffeine intake [78]. In 68 volunteers  consuming a regular daily amount of caffeine, 2 mg/kg caffeine  improved the speed of encoding new information in a categorical search  task [79]. Likewise, in a study comparing 24 non-consumers (20  mg/day) to 24 consumers of caffeine (217 mg/day), there was no  baseline difference between group performances. Caffeine improved  numeric working memory reaction time and sentence verification  accuracy; alertness was also increased, but in general caffeine tended  to improve more the performance in non consumers [80]. In adult  subjects, a higher habitual caffeine intake with no additional  caffeine administration on the day of testing was positively related  to better performance on incidental verbal learning and visuospatial  learning tasks [15]. There was also an association between estimated  caffeine intake and performance in a choice reaction time task, and in  delayed recall of a verbal word learning task [81]. In the former  study, older sub- jects appeared to benefit more from higher caffeine  intake [15], while this was not the case in the latter study [81].  Also, in real life activities, regular caffeine consumption might  benefit cognitive functioning in a non-working population [82].Smith's 2002 reviewAnother good review is provided by Smith (2002, PDF).The main claim relevant to the present question argues that coffee remains beneficial despite habituation:It has been claimed that the positive eects of caeine really reect  removal of negative eects of caeine withdrawal. This view cannot  account for eects observed in non-consumers or nondeprived  individuals. In addition, there is little evidence of caeine  withdrawal impairing tasks which show improvements following ingestion  of caeine.The article is worth reading if you want to read about the relevant empirical evidence and some of the limitations of existing research.References:[11] Rees K, Allen D, Lader M (1999) The influences of age andcaffeine on psychomotor and cognitive function. Psychophar-macology (Berl) 145, 181-188.[78] Smit HJ, Rogers PJ (2000) Effects of low doses of caffeineon cognitive performance, mood and thirst in low and highercaffeine consumers. Psychopharmacology (Berl) 152, 167-173.[80] Haskell CF, Kennedy DO, Wesnes KA, Scholey AB (2005)Cognitive and mood improvements of caffeine in habitual con-sumers and habitual non-consumers of caffeine. Psychophar-macology (Berl) 179, 813-825.[81] Hameleers PA, Van Boxtel MP, Hogervorst E, Riedel WJ,Houx PJ, Buntinx F, Jolles J (2000) Habitual caffeine con-sumption and its relation to memory, attention, planningcapacity and psychomotor performance across multiple agegroups. Hum Psychopharmacol 15, 573-581.[82] Smith AP (2009) Caffeine, cognitive failures and health in anon-working community sample. Hum Psychopharmacol 24,29-34.Smith, A. (2002). Effects of caffeine on human behavior. Food and chemical toxicology, 40, 1243-1255. PDF