We propose Isochronous-MAC (I-MAC) using the Long-Wave Standard Time Code (so called "wave clock"), and introduce cross-layer design for a low-power wireless sensor node with I-MAC. I-MAC has a periodic wakeup time synchronized with the actual time, and thus we take the wave clock. However, a frequency of a crystal oscillator varies along with temperature, which incurs a time difference among nodes. We present a time correction algorithm to address this problem, and shorten the time difference. Thereby, the preamble length in I-MAC can be minimized, which saves communication power. For further power reduction, a low-power crystal oscillator is also proposed, as a physical-layer design. We implemented I-MAC on an off-the-shelf sensor node to estimate the power saving, and verified that the proposed cross-layer design reduces 81% of the total power, compared to Low Power Listening.