Air carrier Captain reported being informed by First Officer and Relief Pilot of a sudden 25 kt increase which resulted in an airspeed exceedance and altitude excursion.

Narrative

I was on break in the crew bunk when the incident happened. Therefore; everything I write here is what was told to me by the First Officer (FO) and Relief Pilot. So as I was told; 130 miles of ZZZZZ in smooth air the aircraft made a sudden 25 kt. increase in airspeed and went into the barber pole. Immediately the Pilot Flying (PF) retarded the throttles to idle and speed brakes deployed. Aircraft lost about 400-500 ft. of altitude; then immediately regained it; aircraft stabilized.This happened in uncontrolled airspace and there was no indication on CPDLC or anything else that there was a conflict. One other thing that the copilots noticed was an immediate temperature change from -57 to -62 at the time of the incident. They said in was like a severe mountain wave; only that we were over the ocean with no mountains nearby. Up in the bunk; I noticed nothing and neither did the flight attendants. I would have never know of the incident if they hadn't told me. Appropriate entry's we're made in the maintenance log.