Prolactin is secreted in a pulsatile fashion with 4 to 14 pulses per day (60% occur during sleep).25 Prolactin secretory pulses begin 60 to 90 minutes after the onset of sleep.26 The amplitude of pulses varies greatly among individuals, with peak levels occurring during the late hours of sleep. Such rises are not clearly associated with any specific stage of sleep. Although some studies have suggested that prolactin varies during the menstrual cycle, the precise nature of this relationship remains unclear. Several investigators have shown that prolactin levels are significantly higher during the ovulatory and luteal phases, particularly at midcycle.27 This midcycle rise may be the result of increased circulating periovulatory estradiol levels. However, other studies have not confirmed this finding. Prolactin is probably not necessary for ovulation, because ovulatory periods may occur in women taking bromocriptine, a medication that suppresses prolactin.