"Most hGH is released at night. Peak spikes of hGH release occur around 10 P.M., midnight, and 2 A.M. The logic behind this night-time release is that most of hGH's effects are controlled by other hormones, including the somatomedins, IGH-I and IGH-II. As a result, the effects of hGH are spread out more evenly during the day." (http://medical-dictionary.thefreedictionary.com/Growth+Hormone+Tests)
"In general, human growth hormone (hGH) secretion follows a circadian rhythm and is secreted in 6-12 discrete pulses per day, with the largest pulse secreted about an hour after the onset of night-time sleep (around midnight for most people)."(http://www.pponline.co.uk/encyc/human-growth-hormone-hgh-901)
"Generally, our growth hormone release is highest during the first part of the night, which is why getting to bed at a decent time for a good nights rest is so important."
"The largest and most predictable of these GH peaks occurs about an hour after onset of sleep. Otherwise there is wide variation between days and individuals. Nearly fifty percent of HGH secretion occurs during the third and fourth NREM sleep stages. Between the peaks, basal GH levels are low, usually less than 5 ng/mL for most of the day and night. " (http://en.wikipedia.org/wiki/Growth_hormone)
I could find only one reference, out of the 8 that I checked, that said the hGH is secreted at specific times during sleep. A few others seemed to hint at it. The rest simply said that it is secreted at intervals after sleep has begun, and did not say you had to be in bed by a specific time. Nevertheless, if one searches more intensively, one is sure to find many more sources to support one position or the other.