  Stellar radiation has conservatively been used as the key constraint to
planetary habitability. We review here the effects of tides, exerted by the
host star on the planet, on the evolution of the planetary spin. Tides
initially drive the rotation period and the orientation of the rotation axis
into an equilibrium state but do not necessarily lead to synchronous rotation.
As tides also circularize the orbit, eventually the rotation period does equal
the orbital period and one hemisphere will be permanently irradiated by the
star. Furthermore, the rotational axis will become perpendicular to the orbit,
i.e. the planetary surface will not experience seasonal variations of the
insolation. We illustrate here how tides alter the spins of planets in the
traditional habitable zone. As an example, we show that, neglecting
perturbations due to other companions, the Super-Earth Gl581d performs two
rotations per orbit and that any primordial obliquity has been eroded.
