  The infrared spectral energy distributions (SEDs) of main-sequence galaxies
in the early universe (z > 4) is currently unconstrained as infrared continuum
observations are time consuming and not feasible for large samples. We present
Atacama Large Millimetre Array (ALMA) Band 8 observations of four main-sequence
galaxies at z ~ 5.5 to study their infrared SED shape in detail. Our continuum
data (rest-frame 110$\rm \mu m$, close to the peak of infrared emission) allows
us to constrain luminosity weighted dust temperatures and total infrared
luminosities. With data at longer wavelengths, we measure for the first time
the emissivity index at these redshifts to provide more robust estimates of
molecular gas masses based on dust continuum. The Band 8 observations of three
out of four galaxies can only be reconciled with optically thin emission
redward of rest-frame 100$\rm \mu m$. The derived dust peak temperatures at z ~
5.5 (38$\pm$8K) are elevated compared to average local galaxies, however, 5-10K
below what would be predicted from an extrapolation of the trend at $z<4$. This
behaviour can be explained by decreasing dust abundance (or density) towards
high redshifts, which would cause the infrared SED at the peak to be more
optically thin, making hot dust more visible to the external observer. From the
850$\rm \mu m$ dust continuum, we derive molecular gas masses between $10^{10}$
and $10^{11}\,{\rm M_{\odot}}$ and gas fractions (gas over total mass) of
30-80% (gas depletion times of 100-220Myrs). All in all, our results provide a
first measured benchmark SED to interpret future millimetre observations of
normal, main-sequence galaxies in the early Universe.
