While sleep health is increasingly becoming important to many people, it is challenging to maintain good sleep quality. Because sleep can be affected by many internal and external factors, which may differ among individuals, it is therefore important to support people to explore and reflect on what factors truly affect their sleep as most existing sleep self-tracking tools mainly focus on showing how individuals slept. In this study, we developed two design probes to support users to investigate personal sleep factors through self-experimenting and reflection. The tangible probe enables individuals to track three personal sleep factors flexibly, while the digital visualization one provides weekly reports of their sleep together with the three tracked factors. We conducted a preliminary field study with five participants over four weeks. The results show that our designs could support users to investigate their personal concerned sleep factors in terms of the causal relationship with the subjective sleep quality.