Schwiegerling and Williams both give the lion's share of the credit (or blame) for night myopia to the eye's substantial spherical aberration, which becomes noticeable only when pupils are dilated.
Schwiegerling estimates that an amateur astronomer whose dark-adapted pupils span 6 millimeters typically will experience a half diopter of night myopia, while the rare stargazer whose pupil can open up as wide as 8 mm may experience a 1 1/2-diopter effect--though both numbers vary widely within the human population.
Scientific uncertainty about its cause notwithstanding, night myopia is "a documented effect," says Bowen--in fact, it has been documented at least since 1789, when Nevil Maskelyne described it to the Royal Society of London.
Finally, consider the possibility that each of your eyes experiences a different amount of night myopia.
If you wear contact lenses by day and experience a measurable amount of night myopia with them on, you may wish to augment your contacts with night-myopia glasses rather than getting a second set of contacts--particularly if you favor extended-wear contacts, or if you are likely to have trouble reading your star charts with souped-up vision.