With summer comes longer days, but nothing we experience locally will compare to the hours of sunlight we'll see in Northern Europe. In fact, we'll be there just after the White Nights of St. Petersburg (read more about it here: http://www.saint-petersburg.com/virtual-tour/whitenights.asp ).
According to TimeandDate.com (http://www.timeanddate.com/worldclock/sunrise.html), we can expect days as long as 18 hours or more. For comparison, here are the sunrise and sunset times for the days we'll be in each port:
Home: Sunrise at 6:38 AM. Sunset at 8:43 PM.
London: Sunrise at 4:49 AM. Sunset at 9:20 PM.
Oslo: Sunrise at 4:09 AM. Sunset at 10:35 PM.
Copenhagen: Sunrise at 4:38 AM. Sunset at 9:51 PM.
Warnemunde, Germany: Sunrise at 4:51 AM. Sunset at 9:41 PM.
St. Petersburg: Sunrise at 4:49 AM. Sunset at 11:10 PM.
Helsinki: Sunrise at 4:20 AM. Sunset at 10:31 PM.
Stockholm: Sunrise at 3:55 AM. Sunset at 9:52 PM.
Back in London: Sunrise at 5:03 AM. Sunset at 9:08 PM.
Sure hope those curtains in the cabin block out the sunlight at least til 6:00 AM.