Quote 4
[…] I asked Dr Seward to give me a little opiate of some kind, as I had not slept well the night before. He very kindly made me up a sleeping draught, which he gave to me, telling me that it would do me no harm, as it was very mild. (19.40)
Even Mina—virtuous, innocent Mina—asks to take some kind of opiate when she has trouble sleeping.
Quote 5
It is a very strange thing, this sleep-walking, for as soon as her will is thwarted in any physical way, her intention, if there be any, disappears, and she yields herself almost exactly to the routine of her life. (7.30)
Lucy is so passive while sleepwalking that Mina is able to redirect her easily enough—as long as she does so physically.
Quote 6
I would have got out to make certain on the point, but some leaden lethargy seemed to chain my limbs and even my will. I lay still and endured; that was all. (19.39)
As Mina is having her blood sucked for the first time, she finds a kind of "lethargy" or passivity creeping over her.