Circadian rhythm, or sleep/wake cycling, is centered in the "suprachiasmatic nucleus" (SCN) within the hypothalamus, and is marked by melatonin levels 2000–4,000% higher during sleep than in the day. A circuit is known to start with melanopsin cells in the eye which stimulate the SCN through glutamate neurons of the "hypothalamic tract". GABAergic neurons from the SCN inhibit the "paraventricular nucleus", which signals the "superior cervical ganglion" (SCG) through sympathetic fibers. The output of the SCG, stimulates NE receptors (β) in the pineal gland which produces N-acetyltransferase, causing production of melatonin from serotonin. Inhibitory melatonin receptors in the SCN then provide a positive feedback pathway. Therefore, light "inhibits" the production of melatonin which "entrains" the 24-hour cycle of SCN activity. The SCN also receives signals from other parts of the brain, and its (approximately) 24-hour cycle does not only depend on light patterns. In fact, sectioned tissue from the SCN will exhibit daily cycle "in vitro" for many days. Additionally, (not shown in diagram), the "basal nucleus" provides GABA-ergic inhibitory input to the "pre-optic anterior hypothalamus" (PAH). When adenosine builds up from the metabolism of ATP throughout the day, it binds to adenosine receptors, inhibiting the basal nucleus. The PAH is then activated, generating slow-wave sleep activity. Caffeine is known to block adenosine receptors, thereby inhibiting sleep among other things.