Andre Borschberg performing yoga in the 3.8 meter cube cockpit of solar powered Solar Impulse 2. Solar Impulse

Andre Borschberg, the 62-year-old Swiss pilot of the solar-powered flight Solar Impulse, has covered half the distance of the world's longest solo duration flight from Nagoya in Japan to Hawaii.

"Already halfway through what is probably the flight of my life. Loving it," tweeted Borschberg, a graduate from EPFL in mechanical engineering and management from MIT.

After 56 hours in the skies above the Pacific, the flight has completed almost 3,900 km of journey and is cruising at 22,000 feet with its batteries all fully charged for the night.

The thermal insulation of the cockpit protects the pilot from extreme temperatures ranging from 40 C to -20 C.

The 3.8 cubic metre cockpit has one seat that doubles up for a reclining berth and toilet.

The seating provides enough space on the long- haul flight with space to stretch legs and do yoga during the five days and nights.

Oxygen supplies, food, survival equipment, a parachute and dinghy are packed into the back of the cockpit.

Flight altitudes depend on the time of day and the pilot starts climbing only when the sun provides enough power. This period is usually two hours from sunrise and ends two hours before sunset.

Solar Impulse 2 is powered by more than 17,000 solar cells built into its 72m-long wings.

High cirrus clouds can limit the amount of energy harvested by the solar cells while trade winds blowing to the west can slow down the craft at night.

Bertrand Piccard, chairman and co-founder of Solar Impulse, will fly the next leg from Hawaii to the US mainland after which the craft will continue across North America, before flying over the Atlantic.

The craft is the successor to Solar Impulse, which notched up a 26-hour flight in 2010, proving its ability to store enough power in lithium batteries during the day to keep flying at night.

The Solar Impulse flight is the culmination of a 13- year dream by the two founders who aim to prove that clean technology works.