  Type Ia supernovae have been proposed to be much better distance indicators
at near-infrared compared to optical wavelengths -- the effect of dust
extinction is expected to be lower and it has been shown that SNe Ia behave
more like `standard candles' at NIR wavelengths. To better understand the
physical processes behind this increased uniformity, we have studied the $Y$,
$J$ and $H$-filter light curves of 91 SNe Ia from the literature. We show that
the phases and luminosities of the first maximum in the NIR light curves are
extremely uniform for our sample. The phase of the second maximum, the
late-phase NIR luminosity and the optical light curve shape are found to be
strongly correlated, in particular more luminous SNe Ia reach the second
maximum in the NIR filters at a later phase compared to fainter objects. We
also find a strong correlation between the phase of the second maximum and the
epoch at which the SN enters the Lira law phase in its optical colour curve
(epochs $\sim$ 15 to 30 days after $B$ band maximum). The decline rate after
the second maximum is very uniform in all NIR filters. We suggest that these
observational parameters are linked to the nickel and iron mass in the
explosion, providing evidence that the amount of nickel synthesised in the
explosion is the dominating factor shaping the optical and NIR appearance of
SNe Ia.
