def _column_tup_to_str(ind):
    """
    Convert tuple of MultiIndex to string.

    Parameters
    ----------
    ind : tuple
        ind[0]: either 'sleep' or 'activity'
        ind[1]: int that is the day number
        ind[2]: bool, True being light, False being dark

    Returns
    -------
    output : str
        Conversion to a single string represnting info in tuple.
        E.g., ('activity', 6, True) gets converted to
        'total seconds of activity in day 6'.
    """
    if ind[0] == 'activity':
        string = 'total seconds of activity in '
    elif ind[0] == 'sleep':
        string = 'total minutes of sleep in '
    elif ind[0] == 'latency':
        string = 'minutes of sleep latency in '
    else:
        raise RuntimeError('%s is invalid MultiIndex' % ind[0])

    if ind[2]:
        return string + 'day ' + str(ind[1])
    else:
        return string + 'night ' + str(ind[1])