In this article the author explains that the aircraft flies at a lower altitude to conserve energy.
During the day, it climbs to its maximum cruising altitude of 8,500 metres to capture the most rays. To conserve energy, it works its way down to 1,500 metres in the evening, and stays there overnight.
How is this possible?
Generally, best range is at high altitude(thin air, less drag), and best endurance is at low attitude(thick air, more lift) and low power.
It makes sense that at night they would have to descend to a best endurance altitude to conserve battery power.
Source: solarimpulse.com
It is basically gliding at night. Not pure gliding but a powered glide - similar to what normal aircraft do on landing approach.
Altitude is energy. We all know this from science class - height = potential energy. Therefore an aircraft that's climbing uses energy and an aircraft that's gliding gains energy (technically it converts potential energy to kinetic energy).
Of course, it has to be done properly since the aircraft has no way of generating power at night. This is why it can't be a pure glide - if it loses too much altitude it won't have enough charge left in the batteries to climb.
The glide requires a lot less energy than maintaining level flight.
In effect, the aircraft uses altitude as a second type of "battery".
"Working down" conserves battery power, as the other answers suggest. But it is also better for endurance to descend, simply because the flight Reynolds number will be higher, and the viscous drag lower.
See this answer for an explanation how the physics behind this works.
See this answer for the Reynolds number effect on a glider. Here the same aircraft is plotted at the same altitude, but with different wing loadings, so all typical speeds increase with wing loading, and so does the Reynolds number. You will see that L/D slightly increases with Reynolds number.
Also, the optimum polar point for minimum energy loss of propeller-driven aircraft is at a high lift coefficient $c_{L_{opt}}$, and this lift coefficient goes up with the wing's aspect ratio $AR$.
$$c_{L_{opt}}=\sqrt{3\cdot c_{D0}\cdot\pi\cdot AR}$$
At an aspect ratio of 20, this might be close to the airfoil's maximum lift coefficient, so flying slowly runs the risk of flying close to stall. Since the maximum lift coefficient goes up with the Reynolds number (after all, the viscous effects diminish and flow separation will be delayed), flying at a low altitude allows to make flying at the optimum polar point for minimum energy loss more safe or even possible.
Altitude is energy. A long shallow dive, from 8,500 to 1,500 meters, recaptures the energy that it cost to climb up during the day. It is completely analogous to charging the batteries with solar power during the day and drawing that energy out of the battery at night.
