  Sunspot light bridges (LBs) exhibit a wide range of short-lived phenomena in
the chromosphere and transition region. In contrast, we use here data from the
Multi-Application Solar Telescope (MAST), the Interface Region Imaging
Spectrograph (IRIS), Hinode, the Atmospheric Imaging Assembly (AIA), and the
Helioseismic and Magnetic Imager (HMI) to analyze the sustained heating over
days in an LB in a regular sunspot. Chromospheric temperatures were retrieved
from the the MAST Ca II and IRIS Mg II lines by nonlocal thermodynamic
equilibrium inversions. Line widths, Doppler shifts, and intensities were
derived from the IRIS lines using Gaussian fits. Coronal temperatures were
estimated through the differential emission measure, while the coronal magnetic
field was obtained from an extrapolation of the HMI vector field. At the
photosphere, the LB exhibits a granular morphology with field strengths of
about 400 G and no significant electric currents. The sunspot does not
fragment, and the LB remains stable for several days. The chromospheric
temperature, IRIS line intensities and widths, and AIA 171 \AA and 211 \AA
intensities are all enhanced in the LB with temperatures from 8000 K to 2.5 MK.
Photospheric plasma motions remain small, while the chromosphere and transition
region indicate predominantly red-shifts of 5-20 km/s with occasional
supersonic downflows exceeding 100 km/s. The excess thermal energy over the LB
is about 3.2x10^26 erg and matches the radiative losses. It could be supplied
by magnetic flux loss of the sunspot (7.5x10^27 erg), kinetic energy from the
increase in the LB width (4x10^28 erg), or freefall of mass along the coronal
loops (6.3x10^26 ,erg).
