Logging actual instrument flight time depends on the weather, not what you can or cannot see. FAR 61.51(g), ?Logging instrument flight time,? says ?(1) A person may log instrument time only for that flight time when the person operates the aircraft solely by reference to instruments under actual or simulated instrument flight conditions.?

The regulations define actual instrument conditions in Part 170, which regulates ?Navigational Facilities,? and gives the definitions for a number of terms used throughout the regulations.

FAR 170.3 says ?Instrument flight rules (IFR) means rules governing the procedures for conducting flight under instrument meteorological conditions (IMC) instrument flight.?

A bit further down the list, FAR 170.3 says ?Instrument meteorological conditions (IMC) means weather conditions below the minimums prescribed for flight under Visual Flight Rules (VFR).?

FAR 91.155 gives the minimum VFR weather conditions, which are the dividing line between VFR and IFR flight. When the weather conditions are below the weather minimums for the airspace you?re flying in, you can log actual instrument time (and you must comply with all the other regulations governing IFR flight, naturally).

So the only time you can log your night flights as actual IFR time is when the weather is below VFR minimums because darkness isn?t a meteorological condition. You can, however, log your night flights, where you?re flying in VFR conditions but can?t see a thing, as simulated instrument time, because the darkness ?simulates? flight in IMC. If effect, darkness and the lack of ground lights is nothing more than a view limiting device you don?t have to wear.